This isn't so hard to test, is it?
Since I'm too lazy to edit rules or events and all... If you put a barbarian ship on a coastal land square using the cheat menu, it will move into the ocean.
Of course, this isn't entirely the same, but I'm pretty sure sea units don't sleep when on land. Ships never do, while land units always automically go to sleep in water...

Now I think of it, this might very well be related to how the units are used. Land units are normally only at sea when loaded on a ship (= asleep). On the other hand, ships are only ever on land when in port (= active). Those two situations might well have dictated what the programmers did in general with the state of ships on land and land units in water.