It all happened so quick. Peri's assault nearly killing the Fishman and then a mysterious blade, presumably Gerrard, finishing it off. He seizes the opportunity to escape the confines of his room.
After a quick look through the hold he spots one Sahuagin that looks different to the others. Perhaps their healer or leader. His thoughts interrupted by the sound of battle, he remembers the cold iron in his hand. Focusing on the pale one he casts Hold Person.
((Upcast to affect the Pale one and the next closest Sahuagin. Wis save DC 15))
Kei steps out over the body of the scaly sea devil, and comes face to face with another spear-wielding sahuagin. But he ignores it for a moment and looks past them.
There are two Sahuagin who appear a little different from the others - one is huge, pale, four-armed(!), wearing a purple breastplate, and bearing a trident (we'll call it "the baron"); the other is greenish and weaponless. They are adjacent to each other, so Kei focuses on both of them:
The Baron: Wisdom save: 6 The green weaponless Sahuagin:5
Both stiffen and freeze in place, staring at you coldly.
This post has potentially manipulated dice roll results.
The battle continues.
Two sahuagin continue to stab ferociously at Oceanus in a blood frenzy: #1 Bite Attack vs Oceanus: 21 Piercing damage: 4 #1 Spear Attack vs Oceanus: 14 Piercing damage7
#2 Bite Attack vs Oceanus: 8 Piercing damage: 4 #2 Spear Attack vs Oceanus: 16 Piercing damage3
Two go after Kei: #4 Bite Attack vs Kei: 12 Piercing damage: 4 #4 Spear Attack vs Kei: 19 Piercing damage9
#6 Bite Attack vs Kei: 6 Piercing damage: 3 #6 Spear Attack vs Kei: 6 Piercing damage3
One goes after Gerrard (who parries) #5 Bite Attack vs Gerrard: 23 Piercing damage: 3 #5 Spear Attack vs Gerrard: 21 Piercing damage2
The bloodbath continues elsewhere: #7 Bite Attack vs Sailor 3: 7 Piercing damage: 4 #7 Spear Attack vs Sailor 3: 18 Piercing damage2
#8 Bite Attack vs Sailor 3: 17 Piercing damage: 5 #8 Spear Attack vs Sailor 3: 18 Piercing damage8
#9 Bite Attack vs Sailor 7: 19 Piercing damage: 5 #9 Spear Attack vs Sailor 7: 21 Piercing damage3
#10 Bite Attack vs Sailor 6: 9 Piercing damage: 3 #10 Spear Attack vs Sailor 6: 12 Piercing damage5
More sahuagin pour down the stairs, even as you hear more dwarven sailors come up from the lower hold.
This post has potentially manipulated dice roll results.
The two remain paralyzed. Another dwarf is killed, even as more sailors come up from below, and more sahuagin come down from above.
Bjorn tries to sting one of the sahuagin (#1) who felled Oceanus: Sting Attack: 5 Piercing damage: 4
But Bjorn is unable to pierce its thick, scaly hide.
Everyone may act (resolved in order: Dwal/Peri, Kei, then Gerrard.)
Edit: Forgot Kei's DC10 concentration checks: Con Save #1: 21 Con Save #2: 13 ...and thus concentration is broken due to Kei's painful wounds, and his two captives are freed from their paralysis. (Although it evidently took a little longer than expected to wear off, however.)
This post has potentially manipulated dice roll results.
Peri:
Peri finds himself trapped in the cabin behind Kei. Gotta get outta this bottleneck, he thinks in frustration, and expends more Ki just to escape into the main hold (Step of the Wind to disengage and move through Kei's and the bottom-left sahuagin's spaces using his Halfling Nimbleness, ending in the space below the sahuagin, one square left of the one he started in). He squeezes through the door behind Kei and somersaults through the legs of the Fishman, slashing at its tender bits -- Do fishmen even have tenders? he wonders -- on the way through. 14 for 6
((I don't suppose I can move into Kei's square, Attack, then use Flurry of Blows, then move to the lower-left square, can I? I assume, even though halflings can move through other creatures' space, they have to attack from an unoccupied square. If I can attack from Kei's square I'll use Flurry of Blows instead of Step of the Wind and I'll roll 2 Unarmed Strikes at the sahuagin I just hit.))
((Edit: I just found the rule in the PHB about moving through a nonhostile creature's space on pg191, and it sounds like I could attack from there so long as I don't end my turn there. I'll roll 2 Flurry of Blows attacks assuming that's the right interpretation; let me know if I'm wrong. #1 10 for 5; #2 17 for 7.))
(Rules time! I did some homework. Weigh in if you know more than I do - and you likely do... 1: You can move through a friendly creature's space, but according to a vague Jeremy Crawford tweet - as the rules are even vaguer - you can't stop there even temporarily to attack. You have to keep moving. At least that's how most people seem to interpret his tweet! 2: Halfling Nimbleness definitely works to get through your opponent in this case - I was about to call you on it, then I realized that halflings are "small" size. For some reason I thought they were medium. Just for reference: the sahuagin are Medium, but the baron is Large. He's a big boy. 3. That makes my third point moot. I was thinking you could just move diagonally... but if we were playing on a grid in the official sense I don't think you can move diagonally if there's a corner like that. However, we're not technically playing on a grid - the grid is just there for general distance estimation and handwavey storytelling reasons. So I'm a little loose with the grid and will maintain my DM discretion powers. But it doesn't matter here so I don't need to rule on it.)
Peri successfully dives between the sahuagin's legs while disengaging from the mob, and spearing the foul creature rather painfully. He winds up atop a crate (in the location you indicated.)
Dwal, meanwhile, moves forward, leaning over Oceanus' unconscious body, and downs the wounded creature in his doorway with his missiles. Two sahuagin dead.
This post has potentially manipulated dice roll results.
Gerrard:
With the doors clear and their front line advancing, Gerrard worries about Kei being exposed. He therefore starts his flurry of stabs at the sahuagin to his west, stepping over its corpse if he manages to take it down to provide some security for Kei on his northern flank.
(Any shots past what it takes to kill his target will be redirected to the enemy to his southeast after he moves. Preparing to parry as a reaction if needed, prioritize the Baron’s attacks if it engages.)
Elegant Rapier: Attack: 11 Damage: 5
Elegant Rapier Attack: Attack: 8 Damage: 11
Main Gauche Attack: Attack: 18 Damage: 7
Rollback Post to RevisionRollBack
Gerrard Feldren - Human Noble in Ghosts of Saltmarsh
Kerric Brightblade - Elven Warrior in "Apocalypse"
"Damn it". He curses the spears that slash across his unarmoured body. His concentration broken he is now acutely aware of just how many Sahuagin are surrounding him. He reaches to his neck and quickly runs his hand along the braided cord til he touches his gods image that dangles over his shoulder. Bringing the black carving back to the centre of his chest he utters a prayer to the god of mischief. A wave of radiant energy flows outward from him, striking the four fish people.
One goes after Kei in a savage blood frenzy; another turns, ignoring Dwal, and follows its comrade's lead, smelling blood in the air: #6 Spear Attack vs Kei: 18 Piercing damage: 4 #6 Bite Attack vs Kei: 7 Piercing damage: 4
#2 Spear Attack vs Kei: 19 Piercing damage: 5 #2 Bite Attack vs Kei: 9 Piercing damage: 5
One goes after Gerrard: #5 Spear Attack vs Gerrard: 14 Piercing damage: 5 #5 Bite Attack vs Gerrard: 12 Piercing damage: 4
One goes after Peri on the crate: #4 Spear Attack vs Peri: 9 Piercing damage: 5 #4 Bite Attack vs Peri: 19 Piercing damage: 2
The bloodbath continues elsewhere: #9 Spear Attack vs Sailor 7: 6 Piercing damage: 3 #9 Bite Attack vs Sailor 7: 18 Piercing damage: 2
#10 Spear Attack vs Sailor 6: 15 Piercing damage: 6 #10 Bite Attack vs Sailor 6: 20 Piercing damage: 4
#7 Spear Attack vs Sailor 11: 15 Piercing damage: 6 #7 Bite Attack vs Sailor 11: 16 Piercing damage: 2
This post has potentially manipulated dice roll results.
In addition to taking serious damage, Kei now glitters with a dim, mystical light.
The surviving dwarves fight back with their spears, as more sailors come up from the lower deck: Sailor #6 Spear attack vs Sahuagin#9: 16 Piercing damage: 3 Sailor #5 Spear attack vs Sahuagin#10: 13 Piercing damage: 7 Sailor #4 Spear attack vs Sahuagin#10: 15 Piercing damage: 4 Sailor #10 Spear attack vs Sahuagin#10: 16 Piercing damage: 3 Sailor #12 Spear attack vs Sahuagin#8: 15 Piercing damage: 4 Sailor #14 Spear attack vs Sahuagin#10: 14 Piercing damage: 8
Bjorn tries again to sting one of the sahuagin (#2) attacking Kei. Sting Attack: 20 Piercing damage: 5
Bjorn manages to inject one of them with his venom: DC 11 Constitution save vs poison: 11
The dwarves fight back and manage to seriously wound one, but they are losing this fight.
Everyone may act (resolved in order: Dwal/Peri, Kei, then Gerrard.) (Kei is in very poor shape - reduced to 4 hp, by my calculations?)
Dwal will pick up his crossbow and a few bolts and load it. He will make his way out of the room and into the main area. "if'n we'd stayed in the room they'd woulda ta come in and get us. We could have bottle-necked 'em," he mumbles. "No use to complain about what ain't."
Dwal will fire a bolt at the closest Sahuagin. Hit: 22 Damage if hits: 7
As a bonus action, he will cast Healing Word on Kei. Heals: 4
The fight is growing desperate as their dwarves hosts are cut down. "Shoulda kept a better watch and maybe we'd have been able to keep at least some of 'em from boardin'!" he growls to himself. He sees by the glow coming off him that Kei is pretty badly hurt. He tries to clear away as many of the attackers from around the bard as he can, launching into a whirlwind of spear thrusts and kicks. (Start with the one adjacent to Peri, moving on to the next most injured one whenever one goes down: 13 for 8, 20 for 4, 11 for 6.) He attempts to interpose himself between his wounded comrade and the Baron (forward of Kei and as far to starboard as he can get as the beasties fall).
Dwal shoots a bolt at the poisoned sahuagin near his doorway. It's still standing, but it's in bad shape.
Peri skewers the nearest one to him with his spear, killing it instantly. Jumping off the crate, he lands a pair of potent blows to the next one. It reels from his punches and is barely standing.
Kei then Gerrard may act.
Key:
2- poisoned, barely standing 5 - seriously wounded 6 - barely standing 8- wounded 9 - seriously wounded 10 - very seriously wounded M=magical one that zapped Kei (uninjured) B=Baron
Kei
It all happened so quick. Peri's assault nearly killing the Fishman and then a mysterious blade, presumably Gerrard, finishing it off. He seizes the opportunity to escape the confines of his room.
After a quick look through the hold he spots one Sahuagin that looks different to the others. Perhaps their healer or leader. His thoughts interrupted by the sound of battle, he remembers the cold iron in his hand. Focusing on the pale one he casts Hold Person.
((Upcast to affect the Pale one and the next closest Sahuagin. Wis save DC 15))
Kei steps out over the body of the scaly sea devil, and comes face to face with another spear-wielding sahuagin. But he ignores it for a moment and looks past them.
There are two Sahuagin who appear a little different from the others - one is huge, pale, four-armed(!), wearing a purple breastplate, and bearing a trident (we'll call it "the baron"); the other is greenish and weaponless. They are adjacent to each other, so Kei focuses on both of them:
The Baron: Wisdom save: 6
The green weaponless Sahuagin: 5
Both stiffen and freeze in place, staring at you coldly.
The battle continues.
Two sahuagin continue to stab ferociously at Oceanus in a blood frenzy:
#1 Bite Attack vs Oceanus: 21 Piercing damage: 4
#1 Spear Attack vs Oceanus: 14 Piercing damage7
#2 Bite Attack vs Oceanus: 8 Piercing damage: 4
#2 Spear Attack vs Oceanus: 16 Piercing damage3
Two go after Kei:
#4 Bite Attack vs Kei: 12 Piercing damage: 4
#4 Spear Attack vs Kei: 19 Piercing damage9
#6 Bite Attack vs Kei: 6 Piercing damage: 3
#6 Spear Attack vs Kei: 6 Piercing damage3
One goes after Gerrard (who parries)
#5 Bite Attack vs Gerrard: 23 Piercing damage: 3
#5 Spear Attack vs Gerrard: 21 Piercing damage2
The bloodbath continues elsewhere:
#7 Bite Attack vs Sailor 3: 7 Piercing damage: 4
#7 Spear Attack vs Sailor 3: 18 Piercing damage2
#8 Bite Attack vs Sailor 3: 17 Piercing damage: 5
#8 Spear Attack vs Sailor 3: 18 Piercing damage8
#9 Bite Attack vs Sailor 7: 19 Piercing damage: 5
#9 Spear Attack vs Sailor 7: 21 Piercing damage3
#10 Bite Attack vs Sailor 6: 9 Piercing damage: 3
#10 Spear Attack vs Sailor 6: 12 Piercing damage5
More sahuagin pour down the stairs, even as you hear more dwarven sailors come up from the lower hold.
Oceanus goes down in a frenzy of spears and teeth.
Kei is also wounded by a pair of spears. (13 damage)
Gerrard, however, fends off his attacker.
The Baron tries to overcome its arcane paralysis: Wis Save: 11
as does its companion: 6
The sailors fight back with spears:
Sailor #7 Spear attack vs Sahuagin#9: 4 Piercing damage: 6
Sailor #6 Spear attack vs Sahuagin#10: 10 Piercing damage: 7
Sailor #11 Spear attack vs Sahuagin#8: 18 Piercing damage: 8
Sailor #5 Spear attack vs Sahuagin#10: 14 Piercing damage: 7
Sailor #4 Spear attack vs Sahuagin#10: 15 Piercing damage: 5
It is utter chaos, a wild frenzy of bloodshed.
The two remain paralyzed. Another dwarf is killed, even as more sailors come up from below, and more sahuagin come down from above.
Bjorn tries to sting one of the sahuagin (#1) who felled Oceanus:
Sting Attack: 5 Piercing damage: 4
But Bjorn is unable to pierce its thick, scaly hide.
Everyone may act (resolved in order: Dwal/Peri, Kei, then Gerrard.)
Edit: Forgot Kei's DC10 concentration checks: Con Save #1: 21 Con Save #2: 13

...and thus concentration is broken due to Kei's painful wounds, and his two captives are freed from their paralysis. (Although it evidently took a little longer than expected to wear off, however.)
Peri:
Peri finds himself trapped in the cabin behind Kei. Gotta get outta this bottleneck, he thinks in frustration, and expends more Ki just to escape into the main hold (Step of the Wind to disengage and move through Kei's and the bottom-left sahuagin's spaces using his Halfling Nimbleness, ending in the space below the sahuagin, one square left of the one he started in). He squeezes through the door behind Kei and somersaults through the legs of the Fishman, slashing at its tender bits -- Do fishmen even have tenders? he wonders -- on the way through. 14 for 6
((I don't suppose I can move into Kei's square, Attack, then use Flurry of Blows, then move to the lower-left square, can I? I assume, even though halflings can move through other creatures' space, they have to attack from an unoccupied square. If I can attack from Kei's square I'll use Flurry of Blows instead of Step of the Wind and I'll roll 2 Unarmed Strikes at the sahuagin I just hit.))
((Edit: I just found the rule in the PHB about moving through a nonhostile creature's space on pg191, and it sounds like I could attack from there so long as I don't end my turn there. I'll roll 2 Flurry of Blows attacks assuming that's the right interpretation; let me know if I'm wrong. #1 10 for 5; #2 17 for 7.))
Dwal
Dwal will move around to where he can see the Sahuagin at the door. Dwal will lash out with another spell Magic Missile at 2nd level.
Damage: 16
(Rules time! I did some homework. Weigh in if you know more than I do - and you likely do...
1: You can move through a friendly creature's space, but according to a vague Jeremy Crawford tweet - as the rules are even vaguer - you can't stop there even temporarily to attack. You have to keep moving. At least that's how most people seem to interpret his tweet!
2: Halfling Nimbleness definitely works to get through your opponent in this case - I was about to call you on it, then I realized that halflings are "small" size. For some reason I thought they were medium. Just for reference: the sahuagin are Medium, but the baron is Large. He's a big boy.
3. That makes my third point moot. I was thinking you could just move diagonally... but if we were playing on a grid in the official sense I don't think you can move diagonally if there's a corner like that. However, we're not technically playing on a grid - the grid is just there for general distance estimation and handwavey storytelling reasons. So I'm a little loose with the grid and will maintain my DM discretion powers. But it doesn't matter here so I don't need to rule on it.)
Peri successfully dives between the sahuagin's legs while disengaging from the mob, and spearing the foul creature rather painfully. He winds up atop a crate (in the location you indicated.)
Dwal, meanwhile, moves forward, leaning over Oceanus' unconscious body, and downs the wounded creature in his doorway with his missiles. Two sahuagin dead.
Kei and Gerrard may act next.
Gerrard:
With the doors clear and their front line advancing, Gerrard worries about Kei being exposed. He therefore starts his flurry of stabs at the sahuagin to his west, stepping over its corpse if he manages to take it down to provide some security for Kei on his northern flank.
(Any shots past what it takes to kill his target will be redirected to the enemy to his southeast after he moves. Preparing to parry as a reaction if needed, prioritize the Baron’s attacks if it engages.)
Elegant Rapier: Attack: 11 Damage: 5
Elegant Rapier Attack: Attack: 8 Damage: 11
Main Gauche Attack: Attack: 18 Damage: 7
Gerrard Feldren - Human Noble in Ghosts of Saltmarsh
Kerric Brightblade - Elven Warrior in "Apocalypse"
Gerrard's second and third strikes connect with the scaly horror, but the foul sea creature keeps coming.
Kei is up. He stands astride a dead sahuagin, even as four of them close in him (two have serious wounds.)
Kei
"Damn it". He curses the spears that slash across his unarmoured body. His concentration broken he is now acutely aware of just how many Sahuagin are surrounding him. He reaches to his neck and quickly runs his hand along the braided cord til he touches his gods image that dangles over his shoulder. Bringing the black carving back to the centre of his chest he utters a prayer to the god of mischief. A wave of radiant energy flows outward from him, striking the four fish people.
Word of Radiance Dmg 7 (Con save 13)
A ripple of burning, holy light emanates from Kei:
Sahuagin 2: CON Save: 15
Sahuagin 4: CON Save: 5
Sahuagin 5: CON Save: 12
Sahuagin 6: CON Save: 7
Three of the four sahuagin are seared; none are defeated although one is in very poor shape.
Oceanus death save: 11
The Sahuagin mass their attack...
One goes after Kei in a savage blood frenzy; another turns, ignoring Dwal, and follows its comrade's lead, smelling blood in the air:
#6 Spear Attack vs Kei: 18 Piercing damage: 4
#6 Bite Attack vs Kei: 7 Piercing damage: 4
#2 Spear Attack vs Kei: 19 Piercing damage: 5
#2 Bite Attack vs Kei: 9 Piercing damage: 5
One goes after Gerrard:
#5 Spear Attack vs Gerrard: 14 Piercing damage: 5
#5 Bite Attack vs Gerrard: 12 Piercing damage: 4
One goes after Peri on the crate:
#4 Spear Attack vs Peri: 9 Piercing damage: 5
#4 Bite Attack vs Peri: 19 Piercing damage: 2
The bloodbath continues elsewhere:
#9 Spear Attack vs Sailor 7: 6 Piercing damage: 3
#9 Bite Attack vs Sailor 7: 18 Piercing damage: 2
#10 Spear Attack vs Sailor 6: 15 Piercing damage: 6
#10 Bite Attack vs Sailor 6: 20 Piercing damage: 4
#7 Spear Attack vs Sailor 11: 15 Piercing damage: 6
#7 Bite Attack vs Sailor 11: 16 Piercing damage: 2
...
Gerrard:
Reaction to parry the first attack: 6
OOC: total of 8 damage gets through.
Gerrard Feldren - Human Noble in Ghosts of Saltmarsh
Kerric Brightblade - Elven Warrior in "Apocalypse"
Everyone receives wounds (although Gerrard may choose to parry), further exciting the bloodthirsty monsters.
With at least two more dwarven sailors the dead, the horde continues its bloody assault.
#8 Spear Attack vs Sailor 11: 23 Piercing damage: 7
#8 Bite Attack vs Sailor 11: 15 Piercing damage: 5
The weaponless green sahuagin fires a bolt of light at Kei:
Bolt of light: 17. Radiant Damage: 19
The baron laughs and holds his trident over his head with all four arms.
Even more sahuagin come downstairs from the deck.
In addition to taking serious damage, Kei now glitters with a dim, mystical light.
The surviving dwarves fight back with their spears, as more sailors come up from the lower deck:
Sailor #6 Spear attack vs Sahuagin#9: 16 Piercing damage: 3
Sailor #5 Spear attack vs Sahuagin#10: 13 Piercing damage: 7
Sailor #4 Spear attack vs Sahuagin#10: 15 Piercing damage: 4
Sailor #10 Spear attack vs Sahuagin#10: 16 Piercing damage: 3
Sailor #12 Spear attack vs Sahuagin#8: 15 Piercing damage: 4
Sailor #14 Spear attack vs Sahuagin#10: 14 Piercing damage: 8
Bjorn tries again to sting one of the sahuagin (#2) attacking Kei.
Sting Attack: 20 Piercing damage: 5
Bjorn manages to inject one of them with his venom: DC 11 Constitution save vs poison: 11
The dwarves fight back and manage to seriously wound one, but they are losing this fight.
Everyone may act (resolved in order: Dwal/Peri, Kei, then Gerrard.)
(Kei is in very poor shape - reduced to 4 hp, by my calculations?)
Dwal
Dwal will pick up his crossbow and a few bolts and load it. He will make his way out of the room and into the main area.
"if'n we'd stayed in the room they'd woulda ta come in and get us. We could have bottle-necked 'em," he mumbles. "No use to complain about what ain't."
Dwal will fire a bolt at the closest Sahuagin.
Hit: 22
Damage if hits: 7
As a bonus action, he will cast Healing Word on Kei.
Heals: 4
Peri:
The fight is growing desperate as their dwarves hosts are cut down. "Shoulda kept a better watch and maybe we'd have been able to keep at least some of 'em from boardin'!" he growls to himself. He sees by the glow coming off him that Kei is pretty badly hurt. He tries to clear away as many of the attackers from around the bard as he can, launching into a whirlwind of spear thrusts and kicks. (Start with the one adjacent to Peri, moving on to the next most injured one whenever one goes down: 13 for 8, 20 for 4, 11 for 6.) He attempts to interpose himself between his wounded comrade and the Baron (forward of Kei and as far to starboard as he can get as the beasties fall).
Dwal shoots a bolt at the poisoned sahuagin near his doorway. It's still standing, but it's in bad shape.
Peri skewers the nearest one to him with his spear, killing it instantly. Jumping off the crate, he lands a pair of potent blows to the next one. It reels from his punches and is barely standing.
Kei then Gerrard may act.
Key:

2- poisoned, barely standing
5 - seriously wounded
6 - barely standing
8- wounded
9 - seriously wounded
10 - very seriously wounded
M=magical one that zapped Kei (uninjured)
B=Baron
Gerrard:
Gerrard spares a worried glance over his shoulder at Oceanus, then quickly returns his attention to the fight before him.
OOC: C’mon Kei, do something awesome!
Gerrard Feldren - Human Noble in Ghosts of Saltmarsh
Kerric Brightblade - Elven Warrior in "Apocalypse"