Aurora's arrow seems wobbly at first but flies true in the end. The serpent reals and looks at the boat, testing its throat. Feeling the chill come back, it knows it can breathe once more on those on the boat. However, it takes a look at Omeros, the one who has harmed it the most, and decides instead to attempt to pull him off the boat with his tail. With this success, it then moves away from the boat and begins to dive underwater, Omeros in tow. As soon as they get underwater, the serpent attempts to bite his newly restrained snack.
As this occurs, the portside (south) serpent attempts to bite Kelnes with a legendary action.
Omeros is now restrained and taken underwater, taking 50 damage (21 bludgeoning and 28 piercing and 1 cold) in the process.
Kelness takes 23 damage (24, but 2 of it is cold so 23)
Kelnes & Isaiah & Chironides are now up. The serpents are about 30 feet underwater and the boat is about 20 feet high, so anyone standing at the edge is about 50-60 feet from them. As such, they have resistance to fire right now and any ranged weapon attack probably has disadvantage. if you feel you have no actions, you can start loading a cannon so it will take less to fire when they come back up.
For those restrained by the serpent, despite the fact you are likely out of reach you may still attack the serpent at disadvantage, as while RAW it is out of reach unless you have a 15 foot reach, part of its body is around you and that would be in reach. Just remember to replace 1d20 with 2d20L1
Rolls
Serpent Breath Weapon Recharge 5-6 - (d6)
(6) = 6
Sea Serpent Constrict vs Omeros AC 18 - (d20+12tohit, 4d10+7bludgeoningdamage)
d20+12tohit : (8) + 12 = 20
4d10+7bludgeoningdamage : (2642) + 7 = 21
Sea Serpent Bite vs AC - (d20+12tohit, 2d12+7piercingdamageplus1d12)
d20+12tohit : (8) + 12 = 20
2d12+7piercingdamageplus1d12 : (912) + (1) + 7 = 29
Advantage roll (for some reason won't let me replace d20+12 with 2d20H1, so here's the other d20) - (2d20H1+12)
(135) + 12 = 25
Sea Serpent Bite vs Kelnes AC 17 - (d20+12tohit, 2d12+7piercingdamageplus1d12colddamage)
d20+12tohit : (4) + 12 = 16
2d12+7piercingdamageplus1d12colddamage : (105) + (2) + 7 = 24
Advantage roll AC 17 - (d20+12)
(11) + 12 = 23