This should be a common sense that you can't move boat through when it is blocked by sea serpent or whale, but I don't see the rule include this situation.
Can a boat move pass through a tile that is occupied by a sea serpent or whale safely? The boat does not stop on the occupied tile.
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.
An occupied boat cannot move through a whale or sea serpent per the following rule (from 30th anniversary edition).
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, its passengers becoming Swimmers (don’t forget that if the sea space contains a Shark or Sea Serpent, these Swimmers are immediately removed from the game).
However, you may move empty ships per the following rule. These ships can move freely through whales and sea serpents.
When a Ship is empty, any player can move it from one sea space to another, as desired (at the cost of one movement per space).
