You're standing at about the middle of the ship, so it's about 10 feet to the edge, and then the serpent is another 20 feet from the edge. So a total of 30 ft.

Fletcher looks at Orimdal "Think you can sail this boat? we need to scram." He looks back to the monster "Why is everything so godsdamn big?" 

With a single deft movement, he slices open his palm with his sword, and once again his blood hardens. The tendrils snake out from the wound, this time trying to wrap themselves around the massive serpent. (DC 13 str save or 0 speed.)

(self inflicted damage 1)

STR save: 6

Alright, so he's going no where.

(I just realized that was only a BA)

Fletcher rushes over to the helm, and tries to turn the rudder so the boat moves away from the creature.

Evi, her laughing fit still raging on, manages to control herself long enough to fire an blast of greenish energy at the creature before finding any place to hide on or below deck outside of the creatures vision.

Attack: 15 Damage: 1 (+3 with Agonizing Blast Eldritch Invocation)

Orimdal runs over to Fletcher at the helm of the boat, "Ya think I know how ta steer this thing ta safety?! I'll give it a go, little fella, but I'll make no promise." Orimdal will take the helm from Fletcher as he turns to send a fire bolt at the sea creature, then focuses his attention on controlling the ship away from the creature and back to the nearest shoreline.

Fire Bolt Attack: 7 Damage: 5

(DM Question - not sure if you want me to make a perception check to see the nearest shoreline, and if that would be an action that will have to wait until next turn. Just let me know what your ruling is.)

(Well it is an action to also use the boat's helm, so you could either do that or steer the boat, but not both)

Fletcher turns the boat 45 degrees away from the creature, and the wind naturally pushes it 20 feet ahead. Evi's Eldritch Blast strikes the creature's underside, but seems to have very little effect. (How long does it have 0 movement?)

(1 Minute, but a str save at the end of its turn can end the effect.)

(Ok, then to continue its turn.)

The serpent thrashes around, sending water flying everywhere. A large wave hits the side of the boat, not causing any damage, but violently rocking it (everyone make a Dex save please).

End of turn STR save: 14

dex save 22

The wave rocks his concentration, and the serpent slips free.

(What are these rolls???? the dice roller loves me)

Turn Order for continued reference: 

Fletcher

Evi

Tender or Ormidal

Kallista

Serpent

Also, can't believe I forgot this, but Evi, since you headed below deck, you can spot a rack of harpoons in a small storage room. These harpoons can be used as weapons (I would say they have spear statistics), and there are 12

Dex Save - 22

Tender hesitates indecisively for a moment before shrugging and pulling out one of her darts, slinging it towards the creature. "We should definitely get out of here," she calls, slightly hysterically, to the others. 

Attack: 9 Damage: 4

Tender's Dart narrowly misses, but the serpent definitely notices it. It glares at you, sneering, saliva dripping from its mouth. It roars and prepares to charge (still three people to go first though, so let's see how much damage it can take in that time)

Dex save: 16

(And got it concerning the Harpoons)

(Doing my first turn since y'all skipped over me)

Kallista shoots an arrow at the creature, then ducks behind some crates in an attempt to hide.

Attack: 8 Damage: Unable to parse dice roll.

Stealth: 18

Dex save: 5

(I think we're still in round 1)

(Nothing's in order I'm really confused at this point)

(Yeah it does seem that the turn order has gotten away from me a bit. Sorry about that.)

Kallista's arrow flies past the creature's head. It doesn't notice, or at least doesn't care. Below deck, Evi is unable to fully keep her balance from the crashing wave and falls over (you are prone).

(Sorry who's turn in combat is it?)