The boat is destroyed if it's occupied.
If you move a Ship containing one or more Explorers into a sea space containing a Sea Serpent or a Whale, the Ship is immediately removed from the game, it's passengers become Swimmers (don't forget that if the sea space contains a Shark or Sea Serpent, these Swimmers are immediately removed from the game).
A similar rules exists for Swimmers.
If you move a Swimmer into a sea space containing a Sea Serpent or a Shark, the Swimmer is immediately removed from the game. Consequently, Swimmers cannot cross that kind of sea spaces.