I have attempted to ask this question on the EA forums in my quest for an answer, but sadly to no avail.
The EA support elves can not assist either, as in game issues are not their forté.
Try this out: Spam grinder tanks against the Rising Sun fleet commander (or other RS AI) on a predominantly naval map. Do your tanks attack their water-based structures? Because you see, mine don't. The option isn't even there, and I'm at a loss as to whether this is something that was just overlooked by the developers, or something I alone am experiencing.
Would someone graciously put my mind to rest on this issue, lest I begin to go insane?