Subs would be a nice supplement to Shore Batteries for base defense, to keep dolts from parking the TG off the end of the runway. Spawn a Sub and try to intercept the TG when it comes back into range.
The spawn points (boat?) would need to be added/built into the terrains.
As subs would be too slow to chase the TG, the counter is simply to move the TG out of the way, but a sub could be fun while the TG is in range.