Yes, it's in the latest version of GAE. Units can be land-based, water-based, or amphibious. There are still some details that need to be ironed out, like how to enable dock-type buildings that can be partially in the water, which would be necessary for producing ships. Currently the only way we have to do it is to have an amphibious unit travel out into the water and make a boat. I'm not sure if producing units on the water works or not, as I haven't tried it, but morphing apparently does.