Bellamy mumbles an incantation as fire begins to form in his hand. The rain coming down turning to steam around his magical fire. He launches the bolt of fire towards the beast but it plummets into the waters before hitting the creature. Possibly the commotion of the ocean disturbing Bellamy's aim. 

The scaled beast twists its way through the water, getting right up to the boat. You see the mouth of the beast emerge once more as its dagger like teeth lunge towards Zane. The quick tabaxi is able to maneuver himself out of the way. The beast then attempts to coil its tail around Bellamy but an arcane spark stops it from grasping him.

The Dragonfly Crew take this opportunity as the beast is distracted to unload their crossbows on the monstrosity. The three members of the crew simultaneously launch their bolts, with two of them puncturing the tough scales. Captain Flame maneuvers the boat to compensate for the massive creature in the waters next to it.

 Thren and Zane. You share initiative. Whoever sees this first go ahead and take your turn.

(Bellamy has mage armor on his AC is 14)

Zane twirls away from the strike of the beast, unsheathing his rapier and taking a stab at it in one fluid motion (using advantage from the owl):

Attack: 26 Damage: 12

He nods to Thren, "You next", giving one bardic inspiration (1d6) for them to use.

Thren up lifted by zane attack shall “right then let’s go you snake” yelling at the creature and will swing for the serpent 

13

damage 4 

As a bouns action I will cast sanctuary on zane 

Zane's rapier sinks deep into the beast. Blood spews from the slash. You look at your arm and notice mixed with the rain and blood, there is an oily sheen. Not just on your arm but covering your whole body. You feel it stiffening for a moment before you muscle through it. Deterring the would be effect.

When Tex’s spear magically returns to his hands, he finally makes out the enormous size of the serpent, or dragon, or whatever is upon them. Something so big they must fight together... maybe he could create an opening for the others? He throws his arms up in reverence, bangs spear with shield to get the serpent's attention, and shouts at it at the top of his lung, “MIGHTY SEA DRAGON! FORGIVE OUR INSOLENCE! ACCEPT MY SACRIFICE!” And he keeps up his arms as to say that he’d welcome its bite at its leisure, but in truth he’s prepared to dodge, or so he hopes, if it even heard him that is.

Rel is up

Through the roaring in her ears from both the storm and the shadowy pact bond of her Hexblade's Curse, Rel feels a deep pang of remorse for the hapless black-haired woman who she has just brutally intimidated and now lies unconscious on the deck.  Her brother...  Why did she bring up her brother?

The blackness in her eyes flickers briefly back to grey as she strives for her father's divine grace to heal the strange young novice sailor:
Bonus Action:  Healing Word (using pact slot):  8 hp healing.

"Hold on to something in the name of the gods if you cannot do your duty as a sailor.  How is abandoning your post and entering the cargo hold going to help you see your brother?"  A disheartening suspicion grips Rel at that moment.  "Is he inside the crate?"

But the din of the storm and ongoing battle soon call Rel's attention and her irises strobe back to inky darkness once more.  Taking advantage of the distraction provided by Tex's groveling, she shifts for a good angle and raises her free hand to fire another beam of black energy at the monster.

Eldritch Blast to hit (with advantage, but rolled low finally):  14
Eldritch Blast damage on hit:  8 + 2 from HBC = 10 force damage and pushed back 10' with Repelling Blast.

Rel grips the door to the cargo hold with her free hand again, not wanting anyone to access the crate with the storm and attack as distractions.

The black haired woman comes to, stumbling about on the soaked deck as she tries to stand. "Yes my brother.... he is in that crate you were hired to protect. I can't lie anymore. Let me see him! Now before we all die to this beast!"

The creature dodges the jet black beam this time. The heavy rain and winds continue on. 

Bellamy is up 

His fur stands on end as was almost taken. He backs up as far as he can then once more hurls a firebolt at the sea creature

Attack: 12 Damage: 4

Then he directs his owl to flyby the seas creature using the help action. Giving advantage to the next person that attacks

As a reaction if hit he will use shield to increase his ac to 19

The firebolt sails off into the rain this time as the serpentile monstrosity lunges his gaping maw at the small feline man tries to escape. Biting into wood it reels back to the waters

The scaled serpent opens it's maw once more to try and eat the cowering fool Tex, but seems to have a problem with these small creature. It then whips its tail up to try and grasp Zane, but the intense storm and tilting boat seem to keep it from doing anything. 

The Dragonfly crew then release another volley of crossbow bolts. They struggle to see through the torrential rain and all of the bolts sail wide. The Captain once again maneuvers the ship through raging waters but his hands slip on the wheel. Everyone sloshes about the boat, Bellamy and Thren, fall prone from the torment of the boat.

Zane and Thren are up in the initiative order. 

Zane attempts a distraction by casting Shatter, aiming it 5 feet behind the head of the creature (to only hit it).

CON 14 save for half damage of: 17 (rolled in game log).

He shouts, "You can do it, Tex!", giving out one more bardic inspiration (d6) as a bonus action.

Thren “yells I’ve had it with you” as he does a 15ft cone of thunder bust around him aiming at the creature 

casting thunder wave at 1st level

5 points of thunder damage (game log) 

con save of 13 to half damage and not be pushed back 10ft

The scaled beast screams "Neeeeyaaahhh" as the shatter pierces it's sense and then almost instantly Thren's force of thunder launches the creature back in the waters.

 Tex is up

While the monster slips back into sea, Tex springs up from his cowering position and hurls his spear at it. It appears he landed a solid hit but instantly the spear disappears and rematerializes in his hand. With spear and shield equipped he stands firm and ready to receive the creature again.