I'm not sure if anyone has noticed, but, when you place a building in the mission editor, in the middle of the sea, it does not sink.
It just maintains its initial position on the surface.
It is even possible to travel to the building by boat and disembark to the building, provided the building is an enterable one.
I can imagine missions like 'Take The Floating Apartment Building', where one team has to defend the building and the other team is invading. Multiplayer of course.
I am using 1.96, so I don't know if this is a recent 'bug'.......I mean 'feature', or if it has been around for a while.
Planck