So so far we have skeletons, sharks, and uh...their variants, so imma try and brainstorm some ideas for more enemy types :P

Skeletons
Sharks
Sirens(these guys/gals would control their opposite sex dragging them towards them until they are shot/dmged at to interrupt them and free yourself)
Naiads(These girls would hang around any island with a pool/lake on it, they would fling globs of water at you to dmg you, and if theres a storm brewing, to make you more attractive to lightning, they only appear in fresh water.)
Sea Serpent(Long giant snake that coils around and poisons any unsuspecting players)
Bat Swarm (These bats would exist in any cave and would divebomb you constantly, it would only take 1-2 hits to disperse them)
Boars( I'm almost positive these will already be a thing)
Ghosts/specters (These would only appear at night on any islands that have a gravesite, they can only be killed with light so get out your lanterns and hold it high, or stay on the ship waiting for the sun/morning light)
Bears(Maybe? i honestly don't know if bears could be a thing or not xD)
Giants/Cyclops(Is that a rock over there? OH SHT THE ROCK MOVED AND ITS SWINGING A HUGE FKING CLUB AT ME!!!)
Spiders(i REALLY hope they don't add this but i thought i would mention it shivers)
Monkeys(Pesky monkeys always dropping down and stealing my gold :/)
Will o wisps (I can't really think of what these guys could do, maybe they could flash really brightly and blind you?)
Crocodile(Can't believe it took me this long to think of them)
Man eating plants (Umm yea cause i wanna be paranoid of every flower on every island)
Creep Vine (These would tangle your feet up and literally "Root" you in place, better get a cutlass or something out)\
Sphinx (Really it could be any statue that gives you 3 riddles and you HAVE to guess correctly or it will attack you)