Morkeleb brings his sword down on the creature’s neck.

But fails in his attack (nat 1)

Being much more accustomed to land based combat, Barnabus struggles with adapting to moving about on a rolling deck. The first bump had jostled him towards the rail. He grabs the railing and struggles to hold on. Just as he thought he was stable, the surface beneath heaves again. His stubby dwarven legs are unable to absorb the shock and the warrior is pitched over the side. [DC=4]

Despite falling into the freeIng water with the beast, Barnabus is up next.

If you end your turn in the water, I’ll need a Con Save as the below freezing water seeps into your clothes. 

On Jar's next turn (as quickly as he can, he would grab hold of the overboard Barnabus and hoist (or aid) his reentry into the boat. 

Barnabus attempts to reboard the ship.

Athletics DC: 19

 Con DC: 14 (incase he's doesn't make it out.)

A 19 is plenty. Barnabas clambers back aboard while Jar takes a glancing blow from the beast. 

Nox is up again.

Getting acclimated to the motion of the rocking ship, Nox plants his feet and fires an eldritch blast at the creature as it swipes Jar.

19 to hit; 10 force damage

Annoyed at the blast that just slammed into it, the plesiosaur turns its attention to Nox. 17 to hit, for 17 Piercing damage. 

Morkeleb is up. 

Seeing his new friend wounded, Morkeleb snarls. “You’ll pay for that, foul beast!” He brings down his sword again, trying to sever the head from the neck.

9 to hit

11 damage if it’s lucky enough to hit

Barnabus grabs his Shield and warpick ands enters the fray. He swings on the creature.

Attack: 16

Damage: 10

Nox is up.

As the creature whips its neck back from recoiling due to Barnabus's swing, Nox launches another Eldritch Blast at its head.

18 to hit; 10 Force Damage if it hits

(Jar’s turn again, before the beast)

Jar settles the boat and watches for just the right opportunity and strikes with his staff. (19 to hit with 9 damage). 

He then punches it solidly (NAT 20 to hit with 7 damage)

Obviously expecting easier prey than what it is getting, the creature reels back after suffering several blows to the head. Blood drips down its face as it huffs out two words, "Drown then." With that, it ducks its head under the water's surface and there is a brief moment of peace. Then the boat rocks as teeth burst through the bottom of the boat. Water starts trickling in through the holes it has made, and the way the boat is rocking, you are sure the creature is still right there, ready to deliver one more destructive blow.

Morkeleb is up.

Morkeleb hesitates—“It can talk?!”

Not seeing a way to hit the creature with his sword any more, and realizing it may be able to demolish their boat, Morkeleb resolves upon a desperate course of action. Dropping his sword and shield so that he can grip the side of the boat, he says to the others in the boat, “hold on to my tail or something, don’t let me fall overboard.”

Then he leans over the side of the boat, sticks his head under the water, and shouts, “Wait, come, speak with us some more!”

Roll a persuasion check for that.

Hopefully Morkeleb can be allowed a quick prayer to cast guidance on himself

persuasion 14

+2 from guidance if allowed, for a 16 total

The boat stops shaking for a moment. Though it doesn't fully raise its head out of the water, the top appears like a crocodile's head, meeting your eyes as you are held off the side of the boat. "Is this, parlay? I'm not supposed to let people fish here, so be quick." it croaks out in a wheezy, coarse voice.

Jar wasn’t sure he heard it talk the first time, but now he is sure. He lost his weapon and waits to see where this goes.