personally, it seems like a clever, but ultimately cheesy exploit.
that’s what I think. I believe this is put into the same category as “all retreats go to the same place” even if it means Infantry moving through enemy forces to get there. I believe these rules were concocted to keep a single battle on the battle board at a time. The concect of assigning ships to naval combat and offshore duty means that forces would have to be physically separated to keep track of them (keep in mind that more experienced players will have no trouble at this, but the rules are geared for newbies).
In my last game versus Trihero, his rolls of 4’s (battleships, bombers, defending fighters) hit 24 out of 27 times. In that case, I would most definately build a submarine.
Cheesy rule indeed, but it is clearly stated and we have to deal with it.