Learn the conditions which spawn a Sea Serpent..... the factors are generally Stormy weather, Rain, Night (although it can sometimes spawn in daytime from personal experience) and I have a theory that it can sense the wind direction and it likes to attack boats downwind (IE making it harder for them to get away).

But unlike other games your corpse will stay there and since Hearth and Homes launched, I believe old death markers remain in place so its possible to go back later and pick it up.

But usually you will see the Serpent swimming upto you, this is your time to check the wind direction and start turning (land mass permitting) and get outta there at full sail, if you wait for it to raise its head out the water and roar..... you are probably too late to out run it unless there is land very nearby