Quite right, the engine doesn't know it's over water but sometimes they do stop! Therefore you need to read up on water "landing" technique and be dressed to survive.
... or believe the book figures that say that if you take the shortest crossing and scrape along the bottom of CAS you're never out of gliding reach of land.
(And it's not always looking at water and ships - on my first crossing I saw neither, as the channel between the cliffs at each side was filled with fog.)