i take a casting pose, arms out palms up.  SLAM!  17 L3 Chaos bolt on 2 13 + 10 Psychic damage.  Mantle of inspiration on Az and Fjolnir, and the 2 injured guards struck for 8 damage.

Round 4: Sahuagin! (Pronounced: Sa-HOO-wa-gin)

Dirk throws such a powerful chaos bolt at #2, its brain melts out of its ears from the psychic damage. Someone is going to have a rough cleanup later. (AZ and Fjolnir may each move up to their speed if they use their reaction due to Dirk's Mantle of inspiration, and they gain 5 temp HP.) The guards will remain where they are but are grateful for the extra HP.

Dustfinger moves up a bit and chucks some eldritch love at #4, after moving his Hex onto it. Attack: 18 Damage: 3 / Attack: 17 Damage: 5 

The Guards move in around the sahuagin so that there's 2 of them on each of the three left. Spear attacks:
Attack: 8 Damage: 4 / Attack: 22 Damage: 2 Guards killed #1
Attack: 12 Damage: 3 / Attack: 6 Damage: 4 Hit #3 for 11
Attack: 19 Damage: 6 / Attack: 4 Damage: 3 Hit #4 for 3

 Captain Miggetter yells back to Fjolnir, "This ship's not made to outmaneuver or outrun, it's made to ram and shoot! Doesn't do much good for things underneath us!"

Az Emberi and Fjolnir are up! If you're taking advantage of Dirk's Mantle of inspiration move, let me know.

Dustfinger's Hex damage: 6

19 Dirk 
17 Dustfinger (C on Hex)
16 Guards
16 Sahuagin Baron
13 Captain & Keel Breaker (Taken 16 damage to underside)
12 Az Emberi 34+5/38
12 Fjolnir 5 Temp HP(C on Hunter's Mark)
12 First Mate & Bosun
11 Sahuagin Priestess (1st: X 3rd: X)(1st: X 2nd: XX 3rd: X)
10 Sahuagin AC 12
#1 DEAD
#2 DEAD
#3 (Taken 11 damage)
#4 (Taken 17 damage, Hexxed by Dusty, STR)
9  Absinthe 23/43
9  Garrett (1st [X][][][] / Inspirations: [X][X][])
8  ???
7  Mary (C on Hex, 3rd: [X][X])
2  Gearbox

I won't move with the mantke thanks for the hp though and on turn I'll move to 4 swing my flame blade

Atk. 27         Dmg.    16

Fjolnir skitters further up the mast to try to get max height and angle on the Priestess, as he sets his concentration on her. "Iffin whatever that thing down under the keel eats us, i'm cutting my way down it's gullet!", He barks to no one in particular. He draws two more arrows, bites the fletching short and looses them at the priestess, hoping his height minimizes water deflection.

Longbow 13 dmg 6  Hunters Mark 6

Longbow 14 dmg 11  Hunters Mark 4

Az Emberi has to move around the guard to hit #4 as the rigging only goes about 10' from the mast in any direction but is able to move in, strike, and retreat away behind him. He would have taken an attack of opportunity if he didn't kill the sahuagin.

Fjolnir Gets one arrow to stick but the other one passes between the priestess' arm and torso.

The First Mate and Bosun fire at #3 and the priestess respectively: Attack: 21 Damage: 4
Attack: 8 Damage: 7
#3 is looking rough but the Bosun missed the priestess.

The Priestess, surfacing to scream in pain and anger at the dwarf she continues to miss that has now pierced her with an arrow,  uses the still floating spiritual trident to attack Fjolnir, along with casting Guiding Bolt at 3rd level:
Attack: 15 Damage: 6 / Attack: 22 Damage: 16 The trident nearly knocks him off balance, then the Guiding Bolt hits him hard with radiant energy. Then she slinks underneath the ship.
Fjolnir: Make 2 separate CON Saves to keep concentration on Hunter's Mark. The first DC 10, the Second DC 12.
Another THUD is heard from below deck:
20 But no further damage is done.

The last Sahuagin snarls and lashes out at one of the guards in a desperate attempt to kill one of the two on either side of him:
Bite: Attack: 12 Damage: 3 
Spear: Attack: 11 Damage: 2 But he misses with both strikes. In a panic, he dives overboard, taking two strikes from the guards around him:
Attack: 12 Damage: 6 / Attack: 4 Damage: 7 One of them catches it with his spear and gashes open its side as it goes over the railing. it bleeds out, clouding the water with crimson.

Absinthe is up! There are currently no targets above water.

For reference, in case anyone decides to pursue underwater: 

A creature can hold its breath for a number of minutes equal to 1 + its Constitution modifier (minimum of 30 seconds).
When a creature runs out of breath or is choking, it can survive for a number of rounds equal to its Constitution modifier (minimum of 1 round). At the start of its next turn, it drops to 0 hit points and is dying, and it can't regain hit points or be stabilized until it can breathe again.
For example, a creature with a Constitution of 14 can hold its breath for 3 minutes. If it starts suffocating, it has 2 rounds to reach air before it drops to 0 hit points.

Attacking Underwater:

When making a melee weapon attack, a creature that doesn't have a swimming speed (either natural or granted by magic) has disadvantage on the attack roll unless the weapon is a dagger, javelin, shortsword, spear, or trident.
A ranged weapon attack automatically misses a target beyond the weapon's normal range. Even against a target within normal range, the attack roll has disadvantage unless the weapon is a crossbow, a net, or a weapon that is thrown like a javelin (including a spear, trident, or dart).
Creatures and objects that are fully immersed in water have resistance to fire damage.

19 Dirk 
17 Dustfinger (C on Hex)
16 Guards
16 Sahuagin Baron
13 Captain & Keel Breaker (Taken 16 damage to underside)
12 Az Emberi 34+5/38
12 Fjolnir 30/54 (C on Hunter's Mark)
12 First Mate & Bosun
11 Sahuagin Priestess (1st: X 3rd: XX)(1st: X 2nd: XX 3rd: XX, Taken 9 damage)
10 Sahuagin
9  Absinthe 23/43
9  Garrett (1st [X][][][] / Inspirations: [X][X][])
8  ???
7  Mary (C on Hex, 3rd: [X][X])
2  Gearbox

Fjolnir clambers along the top of the rigging, reeling, trying to hold concentration; 18   16

Looks like we have to go for a swim to finish this.  Do we think these things are intelligent enough to negotiate at this point?  Seriously, does someone want to dangle me over the side for a little powow fireside chat with Ariel's over caricature of an aunt?  I mean we get it, you are the evil antagonist of the story but it wouldn't hurt to put on a little make up and do some situps once in a while AMIRIGHT? While I look to everyone for affirmation I am definitely looking at Mary, making that "Come on, for reals though right?" face.

Absinthe begins patrolling the waters from the safety of the ship. Looking at the damage to the crew and ship both on and off deck. "I'm not a great swimmer, I don't know how much help I could be under water." 

Quote from Evildrive >>

Looks like we have to go for a swim to finish this.  Do we think these things are intelligent enough to negotiate at this point?  Seriously, does someone want to dangle me over the side for a little powow fireside chat with Ariel's over caricature of an aunt?  I mean we get it, you are the evil antagonist of the story but it wouldn't hurt to put on a little make up and do some situps once in a while AMIRIGHT? While I look to everyone for affirmation I am definitely looking at Mary, making that "Come on, for reals though right?" face.

Captain Miggetter chimes in, "They're intelligent enough, lad, but too savage to care about yer lives. They don't speak common, and they'd just assume bite ya than look at ya. We kill them all, or we try to make it two more days with a giant battering ram trying to sink the ship!"

Absinthe peers over the edge, but doesn't see anything. The spot under the boat being rammed is only barely visible from inside, but hasn't slowed down.

Garrett holds a Vicious Mockery for any creature that would show itself, but doesn't do anything else.

BOOM! 22 The boards flex upward below deck and return to their initial position.

Mary says, "I'm not going down there." She holds an Eldritch Blast.

Gearbox follows Garrett's lead and waits for something to show its face.

Dirk is up!

19 Dirk 
17 Dustfinger (C on Hex)
16 Guards
16 Sahuagin Baron
13 Captain & Keel Breaker (Taken 96 damage to underside)
12 Az Emberi 34+5/38
12 Fjolnir 30/54 (C on Hunter's Mark)
12 First Mate & Bosun
11 Sahuagin Priestess (1st: X 3rd: XX)(1st: X 2nd: XX 3rd: XX, Taken 9 damage)
10 Sahuagin
9  Absinthe 23/43
9  Garrett (1st [X][][][] / Inspirations: [X][X][]) Holding Vicious Mockery
8  ???
7  Mary (C on Hex, 3rd: [X][X]) Holding Eldritch Blast
2  Gearbox Holding Attack

Maybe it's a good day to die?  I dive in to get a bead on this assailant, and light it up.  L1 pink Faerie fire.  10 Wild magic.

Dirk dives down to discover the source of so much banging. The ship's hull is not deep, and he very quickly catches a glimpse of it.
Dirk, and anyone who jumps into the water on their turn:

A flash of pink is seen from the depths. DEX Saves: 20 / 3 / 18  3 nopes.

Dustfinger sighs and dives in as well. Attack: 26 Damage: 8 + 2
Dirk:

 The Guards do not follow suit, but hold for anything they might see.

 The ship continues on, albeit slower, and the captain stays the course.

Az Emberi and Fjolnir are up.

19 Dirk 
17 Dustfinger (C on Hex)
16 Guards (Holding Attacks)
16 Sahuagin Baron
13 Captain & Keel Breaker (Taken 16 damage to underside)
12 Az Emberi 34+5/38
12 Fjolnir 30/54 (C on Hunter's Mark)
12 First Mate & Bosun
11 Sahuagin Priestess
#1 (1st: X 3rd: XX) (Faerie Fired)
#2 (1st: X 2nd: XX 3rd: XX, Taken 9 damage) (Faerie Fired)
10 Sahuagin
9  Absinthe 23/43
9  Garrett (1st [X][][][] / Inspirations: [X][X][]) Holding Vicious Mockery
8  ??? (Taken 5 damage, Faerie Fired, Hex STR)
7  Mary (C on Hex, 3rd: [X][X]) Holding Eldritch Blast
2  Gearbox Holding Attack

Az emberi leaps over the side and transforms into a reef shark mid air.

Fjolnir sighs miserably, draws another two arrows, bites the fletching off and jack knives into the water, loosing them at his still marked quarry-

Longbow 11 dmg 6  Hunters Mark 2

Longbow 18 dmg 9  Hunters Mark 4

Az Emberi gracefully bounds over the railing and magically becomes a shark mid-fall, belly-flopping into the ocean with a giant splash and diving a few feet.

Fjolnir joins the underwater party and fires a pair of arrows at the spell-slinging fish-person, and actually hits with both arrows. (Dirk's Faerie Fire would cancel out your disadvantage) She's looking rough.

The First Mate and Bosun look at each other, then the captain, who nods, and they both jump off the side of the ship. They nearly shit themselves, then fire their arrows at the water-breathing beast. Attack: 18 Damage: 6 / Attack: 8 Damage: 6 one arrow catches it's side and grazes it.

The Priestesses simultaneously shout something in fish-speak, and point at Dirk and Dustfinger. The Giant Shark, in response to this, turns in their direction... Then they each release a blast of radiant energy.
#1 vs Dustfinger: 7 On Hit: 17
#2 vs Dirk: 14 On Hit: 15 
#1 Crit: 19

Absinthe is up.

19 Dirk 18/32
17 Dustfinger 11/33 (C on Hex)
16 Guards (Holding Attacks)
16 Sahuagin Baron
13 Captain & Keel Breaker (Taken 96 damage to underside)
12 Az Emberi 34+5/38
12 Fjolnir 30/54 (C on Hunter's Mark)
12 First Mate & Bosun
11 Sahuagin Priestess
#1 (1st: X 2nd: X 3rd: XX) (Faerie Fired)
#2 (1st: X 2nd: XX 3rd: XXX, Taken 29 damage, looks rough) (Faerie Fired, Hunter's Marked)
10 Sahuagin
9  Absinthe 23/43
9  Garrett (1st [X][][][] / Inspirations: [X][X][]) Holding Vicious Mockery
8  Giant Shark (Taken 12 damage, Faerie Fired, Hex STR)
7  Mary (C on Hex, 3rd: [X][X]) Holding Eldritch Blast
2  Gearbox Holding Attack

"OH! That's right I can swim!" 

Absinthe ssteps over the railing and Preforms what he hopes is a flawless dive. 

Athletics: 25

He enters the water as if superman flying into the ocean. 

Absinthe dives in but doesn't have much time to do much else. (I'll say you hold an attack since you didn't take an action)

Garrett sighs and dives in after all of you, losing his held action since nothing showed itself to strike. His eyes widen at the sight of the giant shark, and he gathers his wits to somehow, even while underwater, throw a Vicious Mockery at Sahuagin Priestess #2, seeing her less than perfect physical state. WIS Save DC 13 or 3 psychic and (D) on next attack roll: 17 ...to no effect.
Garrett then uses his last Bardic Inspiration on Dirk.

The Giant Shark, frenzied by the scent of blood, speeds through the water at Dustfinger. Attack: 29 Damage: 41 Catching the warlock in its mouth, the shark tears into flesh, and Dustfinger passes out. He is now bleeding out and drowning and, needless to say, no longer concentrating on Hex.

Mary, still not keen on jumping into the sea, looks over one side, and upon seeing nothing, moves across the ship to look over the other. She gasps and then throws a pair of Eldritch Blasts at the Giant Shark, after moving her Hex to it.
Attack: 18 Damage: 7 + 5 / Attack: 8 Damage: 2 + 2 
Thanks to the pink sparkle around the shark, she rips into it with both bolts of force, doing 24 total damage, yet it still looks unphased.

Gearbox sees Mary attacking something and rushes to aid, also preferring not to get saltwater in places unseen, which would probably actually be detrimental to his inner workings. He lobs a javelin at the beast's head. Attack: 19 Damage: 3 The shark is now adorned with a javelin on the side of its face but seems to pay no mind.

Dirk is up.

For orientation purposes, I placed the shark and priestesses atop the ship, but this is merely their underwater position. And yes, Priestess #1 is riding the shark!

19 Dirk 18/32 (d6 Bardic Inspiration)
17 Dustfinger 0/33 (Unconscious, Drowning, Death Saves: [][][]/[][][])
16 Guards (Holding Attacks)
16 Sahuagin Baron
13 Captain & Keel Breaker (Taken 96 damage to underside)
12 Az Emberi 34+5/38
12 Fjolnir 30/54 (C on Hunter's Mark)
12 First Mate & Bosun
11 Sahuagin Priestess
#1 (1st: X 2nd: X 3rd: XX) (Faerie Fired)
#2 (1st: X 2nd: XX 3rd: XXX, Taken 29 damage, looks rough) (Faerie Fired, Hunter's Marked)
10 Sahuagin
9  Absinthe 23/43 (Holding melee attack)
9  Garrett (1st [X][][][] / Inspirations: [X][X][X])
8  Giant Shark (Taken 41 damage, Faerie Fired, Hexed INT)
7  Mary (C on Hex, 3rd: [X][X])
2  Gearbox

I swim to dustfinger, l1 cure wounds 5. Mantle of inspiration on me, dustfinger, As and fjolnir if possible.  I swim out up 25ft and to the left 5ft to make  this creature more exposed.  Get  outof the water sharkbait!  I call out to Dustfinger to my best ability. 

Round 6: Sahuagin! (Pronounced: Sa-HOO-wa-gin)

Dirk rouses Dustfinger with a touch of healing and swims away after granting himself free movement and bonus HP.

Dustfinger uses his granted movement and new lease on life to attempt to make it back to the ship. He uses his turn to climb up the side and back onto the deck. He fires off two blasts of eldritch energy: Attack: 24 Damage: 1 / Attack: 26 Damage: 2 

The Guards all hold position, fearing their armor would drag them down if they jumped in, and watch helplessly from the deck.

Captain Miggetter yells, "Halt! We're not leavin' anyone behind!" the ship lurches as the oars are dipped in the water to stop its advance.

Az Emberi and Fjolnir are up!

19 Dirk 18/32 +5 (d6 Bardic Inspiration)
17 Dustfinger 10/33 +5
16 Guards (Holding Attacks)
16 Sahuagin Baron
13 Captain & Keel Breaker (Taken 96 damage to underside)
12 Az Emberi 34/38 +5
12 Fjolnir 30/54 +5 (C on Hunter's Mark)
12 First Mate & Bosun
11 Sahuagin Priestess
#1 (1st: X 2nd: X 3rd: XX) (Faerie Fired)
#2 (1st: X 2nd: XX 3rd: XXX, Taken 29 damage, looks rough) (Faerie Fired, Hunter's Marked)
10 Sahuagin
9  Absinthe 23/43 (Holding melee attack)
9  Garrett (1st [X][][][] / Inspirations: [X][X][X])
8  Giant Shark (Taken 48 damage, Faerie Fired, Hexed INT)
7  Mary (C on Hex, 3rd: [X][X])
2  Gearbox

"Blrrblrrblrarblrrbl!", Fjolnir states, aggressively, taking two more shots at the priestess.

Longbow 28 dmg 7  Hunters Mark 1

Longbow 15 dmg 4  Hunters Mark 6

dmg 8 

Shark Az attacks the weakest priestess still standing (swimming? Floating?) 

Atk.    21        Dmg.      9