On second thought having a boss that you could fight in the ocean is a way better idea then my prior one.
The boats that we have currently are perfect for multiple players and using fires or fridges during battle at sea.
So im definetly thinking of a boss that would have tentacles that would pop out one by one until its head comes out, wich would have a design of a decaying horses or dragons skull with bits of moss and sea weeds on it. It would have attacks that would make you fall asleep and an AOE attack that would put holes in your boats
Emerald Scales- can be used to craft a tape that would make a boat have no leaks (Sort of like dragonfly scales just for boats)
Leviathan's Jaw- It would be able to destroy those pesky undestroyable rocks and give your boat extra speed as it charges