I've never played with cargo-ships, so I took a look.
I'm not opposed to the idea and, I would want to do it in a way that meshes well with the gameplay. It seems that with a port you'd be near land anyway. Other combinators used for logic, inserters, to load and unload, etc... Don't all these things require land too?
I'm not sure I see a large advantage to allowing this single tile entity to be placed on water. Again, I don't play carg-ships and may not see the use case.