They Spawn on Oceans Biome, wich is the deep sea.

They spawn rarely, they have higher chances of Spawn during storms.

When they freshly spawn they will usually Spawn in your back, with sticking their head out of the waters and growling at you, sometimes they will emerge in front of you and you can pass them or ram them caussing damage to both them and the ship.

They are fast and do decent damage, its impossible to escape with a raft, you have a slight chance with a Karve depending on your manoeuvring skills and the wind.
You can totaly leave them in the dust if you have a Longboat.

They will usually flee if their HP is under 50-40%, be mindfull though that if they flee from been too wounded, they will not despawn, chances are that if you pass by the same spot it fled, later on, you'll encounter it again.( at this moment you'll see that contrary to when they freshly spawn, they don't stick their heads out to growl at you, they're simply swimming towards you, so they are somewhat harder to spot)

The tactic is to use flint arrows at minimum, Fire arrows doesn't really work, since the thing is int eh water, and the water will put out the flames.

Ice arrows & Obsidian arrows or above are great.

Wound it enough that he would rather flee than fight, then if you have one, use a Harpoon, to capture it, then sail to shore and drag it out of the water to kill it.

You can kill it in the water, but you'll only get the meat, the scales and trophy(when it drop) will sink at the bottom of the water.

Harpoons are made using Chitin, that you can mine on the back of Giant sea turtles that people call either Leviathans or Krakens, be carefull that even tho they don't move around, once you start mining the Barnacles on their back, they'll occasionaly shake, and you'r ethen on a timer.

Once the timer's up, the Leviathan will sink in the sea, so be carefull to have enough stamina to swim back to your ship.

Since Serpents are from a different Biome, all other creature will attack it once you get it to shore.