The problem on water is that not only does it make no sense logically but you can't guarantee that the water has port access of any kind. I see players building towers on tiny ponds only a few squares in size where there is no possible way to bring any sea/land units.
Better to just have towers on land only or if you want to allow towers in water then force them to be constructed by a unit in a boat (no flying/sea riding hero/unit) which guarantees there is a port of some kind.
KGB