Shipwrecked in Saltmarsh

Dain grabs his maul, screams a war cry and charges the harpoonist fighting the captain!

Bonus Action: Rage
Move:  35 ft. towards the creature.
Action:   
Recklessly Power Attack 1:  Attack: 16 Damage: 21
Recklessly Power Attack 2:   Attack: 11 Damage: 27
If either hits, extra Divine Fury damage:  7 radiant damage extra on first hit.

Dain scrambles up the listing ship, sprints across the deck, and dives over the side, wildly swinging his powerful maul overhead. He belly-flops in the surging and bitter cold surf, missing the huge, one-eyed beast entirely.

One-Eye responds, lunging at the splashing dwarf with his newly-returned harpoon in one hand and an open, clawed hand at the other:
Claws Attack vs. Dain: 23   Hit: 7 slashing damage.
Harpoon Attack vs. Dain: 26   Hit:12  

The harpoon pierces clear through Dain's shoulder. Crit damage: 6 piercing damage.

Molo is up next (or is he continuing his ritual?)

Molo continues his ritual. He is about one minute away.

The two more distant harpoonists whose weapons stuck the ship begin to pull, with incredible strength, on their lines.
Strength checks: 17 and 19  

The lines go taut, but eventually, their harpoons are wrenched out of the wood, having done little damage; If they were trying to move or tip the vessel, they failed.

The third harpoonist swims closer and throws his weapon at Fianna:
Harrpoonist #3 vs Fianna: 17  Hit: 9 piercing damage.

Another hit, this time on her left arm. Fianna must succeed on a Strength contest or be pulled up to 20 feet toward it.
Harpoonist strength contest: 22 

Fianna must make a DC16 strength check.
The dwarves on board the ship scramble for their spears.
Everyone may act. 

Fianna Strength Check:  6

     Fianna is pulled 20 feet towards her foe...  (I'm assuming, not having seen the roll.). Which foe though?  If it is the first one, I think 20 feet brings Fianna right next to it?

     As a reaction to the damage, Fianna points at the harpoons which struck her and unleashes her Hellish Rebuke upon his rudeness!  Dex save VS DC 15 for half of 13 fire damage....

(He wasn't quite done...)  Having pulled her in, the creature snaps at Fianna with his massive jaws:
Bite Attack: 14  Hit: 8 piercing damage

But he finds himself surrounded by sinister, arcane flames. DC15 Dex save: 20

All of the creatures now have serious wounds, but none are near death quite yet.

Everyone but Fianna may act next.

(Well it was just her reaction, which didn't require him being done!) lol

     As a Bonus action on her turn, Fianna takes a careful Misty Step away from the sea creature and teleports 30' south back onto the ship...  She then will use Eldritch Blast to shoot two beams of energy at the creature she just escaped.

EB1 Attack: 27 Damage: 9

EB2 Attack: 13 Damage: 9

Fianna teleports back to the stern of the boat, and then fires off a pair of crackling beams of violet energy at her new attacker. Both hit, although the first pegs her attack square in its left eye.

It howls in pain, then bellows in a language you do not understand, "မင်းကငါ့ကိုအံ့သြတဲ့မျက်လုံးနဲ့ငါ့ကိုရိုက်တယ်။!" Still, however, it remains aggresive.

Everyone except Fianna may act next.

Dain growls in pain as the harpoon impales him, but he mindlessly continues to pound on old-one-eye!

Action:
Recklessly Power Attack 1:  Attack: 18 Damage: 26
Recklessly Power Attack 2:  Attack: 9 Damage: 18
Note:  Dain is granting Advantage to all attacks against him till his next turn.
Divine Fury:   Extra 4 radiant damage on the first hit this turn due to rage.

Treading water, Dain takes down the one-eyed creature with two wild blows of his maul. It sinks beneath the waves, its skull crushed. It will not surface again.
He may swim the few feet to the adjacent creature and get in a bonus slipe.

But the captain gets there first:
Captain's blade #1 vs. harpoonist #2: 6 Hit: 4 slashing damage
Captain's blade #2 vs. harpoonist #2: 21 Hit: 8 slashing damage

 The captain scores a serious wound on harpooinst #2; Dain may also attack it with his bonus attack.

Kelnan, Molo, and Katernin may act as well.

Dain swims the few feet to the one now fighting the captain and tries to take it out:

Move:   Swim to the next bad guy
Bonus Action:  Reckless Power Attack: 21 Damage: 27
Note:   Still granting Advantage and still raging.

Kelnan adds to the volley against Fianna's attacker with a Ray of Frost.

Attack: 26 Damage: 13

Sloshing angrily through green, turbulent waves beating against the side of the grounded vessel, Dain takes down the second creature next to the captain. Only the three distant harpoonists remain.

Kelnan fires a beam of blue light at the leftmost one. It catches the creature square in the throat, and it is in distress. It is in very poor shape, but it still lives. "ငါ့မျက်စိ!! ငါ့မျက်စိ!!!!" it bellows, in a voice that does not sound of this world.

Molo, and Katernin (flying above), may act next.

Molo continues his ritual.

Four of the crew fling spears at the distant creatures, but it's a long throw:
Crew Spear Attack vs. #3: 7   Hit: 5 piercing damage
Crew Spear Attack vs. #4: 7   Hit: 7 piercing damage
Crew Spear Attack vs. #5: 6   Hit: 4 piercing damage
Crew Spear Attack vs. #5: 9   Hit: 5 piercing damage

Even at long-range, two manage to strike the rightmost creature. It is seriously wounded.

Nevertheless, they continue. All three close in; one toward Fianna and two toward the captain and Dain.
They launch their harpoons again:
  #3 Harpoon Attack vs crewman near Fianna: 19  Hit: 10 piercing damage (and must succeed on a DC 14 Strength contest  or be pulled 20 feet toward it.)
  #4 Harpoon Attack on Captain: 10 Hit: 7 piercing damage (and must succeed on a DC 23 Strength contest...)
  #5 Harpoon Attack on Captain: 16 Hit: 10 piercing damage (and must succeed on a DC 6 Strength contest...)

The crewman near Fianna is impaled; his lifeless body is pulled into the black crashing waves over the wreck.

The captain is similarly struck. He fights back - Strength contest 13 -

...but fails against the superior strength of these huge monstrosities.  It reels the Captain in and delivers a savage raking of claws:
#4 Claws Attack vs. Captain: 10  Hit: 9 slashing damage.

 It fails to sink its claws into the tough master.

Everyone may act next.

     Fianna casts Cure Wounds on herself for 9 and otherwise just tries to not get harpooned again…

     Fi’Finella however flies over to the one struggling with the captain and attempts to harass and distract the creature to try to help the captain or whoever attacks it next…

Dain decides to stick with the captain and so swims the few feet to the one that reeled him in.

Move:  swim to the middle one.
Action:
Recklessly Power Attack 1:  Attack: 20 Damage: 22
Recklessly Power Attack 2:  Attack: 14 Damage: 21
Divine Fury on the first hit:  6 radiant damage
Note:  Granting Advantage and still raging.

Should his attacks kill that harpoonist, and should he have enough movement left he will:

Move:  swim to the injured one on the right
Bonus Action (due to GWM, only if he's killed one):  Recklessly Power Attack: 21 Damage: 28