vital rapids
something like subnautica's ghost leviathan. but it could be something like if you get far enough out, at a certain point theres no going back. the fog closes in, all music fades to silence, all you hear is ambient ocean noises or water and then something approaching than when it reaches you, you die instantly and are teleported back to the spawn point. if you want to be real fancy add a fnaf type jumpscare to it. if you dont want to kill the player just tp back to spawn through a portal after the scare.