Qivys is down.

Leaning far over the bow of Peri's drifting little ship, and aiming for the pincer joints, Gerrard twice manages to pierce the armor of the creature holding Qivys, in an attempt to break its grip. DC17 STR check: 24 

Meanwhile, Peri manages, with all four well-aimed blows, to crack the creature open. It collapses into the black waters, dead. (Note: That fourth blow was indeed needed to drop it.)

Qivys falls into the sea as well. One creature remains.

Boko and Thwack may act next.

Thwack 

"Snarp!" Thwack grunts in excitement as Peri delivers the final blow to another chuul. Scanning for more targets he catches a glimpse of a dwarf splashing limply into the water. "Me hate the wetter" he exclaims but without hesitation he launches into the turbulent sea and swims toward Qivys. With surprising speed he reaches the sinking dwarf and grabs out, hoping to latch on.

On the surface - Just as Thwack hits the water, a familiar black tentacle materialises next to the remaining creature. It lashes out at the crustacean.

A - Dive and swim to catch Qivys

BA - Tentacle of the Deep summon and attack. Attack: 13 Damage: 5

Boko looks around at everyone dropping around her and shakes her head to clear it and casts aura of vitality and heals Qivys for 6hp. She ducks her head underwater to see what's going on, and then pops back up. (Can post more color later.)

Thwack manages to stabilize Qivys in the water and keep him from drowning, but it will take another action (and strength or dex check) to get him back into the boat. Meanwhile, his summoned tentacles harass the remaining chuul.

Boko casts her healing aura, and Qivys awakens in Thwack's arms. Looking below, Boko sees nothing under them but death and gore; no additional dangers. Pieter and Jax load and aim the ballista once again, as the captain remains paralyzed in the bottom of the vessel.

The creature grabs for Thwack, next:
Pincer Attack #1 vs Thwack: 26  Hit: 8 bludgeoning damage and Thwack becomes grappled.

Grabbing the poor kobold and squeezing him, the chuul comes in with its squirming pink tentacles:
Thwack: Make a DC 13 Constitution saving throw or be poisoned and paralyzed. 

DC13 CON Save for the captain: 4

Everyone may act!

(Crit damage on Thwack: +8)

The boats continue to drift in the wind. Gerrard and Peri are out of melee range.

Thwack

"Sorry I ate your brother!" Thwack manages to apologise to the oversized lobster as he trades place with Qivys. Now grappled, he squirms against the incoming tentacles.

CON save 12

Thwack 

The tiny kobold wriggles into a less prone position just as the claw snaps around his waist.

(Class Feature - Uncanny Dodge. Halving the damage taken)

Gerrard:

”Really?” exclaims Gerrard with exasperation.  He drops his rapier point first in the deck where it sticks with a resonant *Twang* and leaps onto the chull’s back, stabbing it twice with his flaming dagger and trying, once again, to get it to drop a friend out of its claws!

Fiery Dagger:  Attack: 21 Damage: 19

Fiery Dagger:  Attack: 28 Damage: 11

Extra Damage on first hit and disarm attempt (DC 17 Strength to counter):  1

For just a moment Peri hesitates, as the odd idea pops into his head of the megacrawdad squeezing Thwack hard enough to squeeze Kei out. He shakes off the crazy idea and instead curses the strength of the creature, hoping Thwack will be able to wriggle out of its claw. Then he fires off an eldritch blast into it.

EB #1: 11 for 6 force + 3 necrotic
EB #2: 21 for 5 force + 5 necrotic

Qivys sputs and sputters out water, cursing as he does.
"I'm startin' ta hate tha sea and all that's in it." he mutters.

Seeing the kobold in the monsters clutches, Qivys puts himself in a position to act. If he can, he will stretch out his hand and says:
"Choke on eldritch power." and casts Magic Missile , using his last 3rd level slot.

Magic Missiles (5): 13

Boko reaches out again and casts frostbite on the creature for 10 cold damage and disadvantage on its next attack, on a failed CON 17 save. 

She also sends healing magic from her Aura again to Qivys, 8hp. Rolled with avrae. 

Thwack 

"Aarrrggghhh" Thwack cries out as the pincer squeezes him tight. He wriggles and twists against the claw, trying to find a position to slip from its grasp.

BA - Racial Feature - Draconic Cry (giving everyone Adv on attack rolls for 1 turn)

A - Acrobatics check to escape the grapple 13

Thwack dodges the poisonous tentacles but is still firmly grappled.

Then Gerrard leaps upon the chuul, and manages to pierce its armor, trying to pry Thwack loose. DC17 STR check or drop Thwack: 13

Peri releases a pair of beams of crackling energy, but both go wide he struggles to steady himself on the drifting little boat. (Nevertheless, he silently curses the chuul's prodigious strength.)

Qivys, on the other hand, does not miss with his volley of arcane glowing darts.

Boko releases a sheen of numbing frost upon it. DC 17 CON save: 18

Thwack is dropped into the water. (He may still have his action.)

The remaining chuul is in quite bad shape. 

Pieter fires the ballista. Bolt Attack: 24. Hit: 9 piercing damage.
Jax immediately reloads it.

The captain struggles to overcome her paralysis. DC13 CON Save: 4

Thwack may act next. 

The chuul head explodes as Pieter's massive bolt goes right through it. Crit damage: +15

The thing collapses into the water, dead. Pieter smiles and nods. The captain sits up and coughs.

Combat is over.

Gerrard:

"Nice shot, Pieter!" calls Gerrard in astonished praise...

Pushing off the decapitated chull with vigor, Gerrard swims back to the small boat and pulls himself on board.  Rinsing off his weapons, he dries and sheathes them, then says to Peri, "They look like crabs or lobsters, Captain Fenwader...Do you think one might make a good meal while we wait out the storm?  I think we could all use a meal and a rest before heading ashore for any scouting."