Doesn't every aircraft/vehicle have pros and cons though? It will have a place in aces high, even if it is just a port defence role.
I would vote on adding the Submarine, Pro's of it would be as long as the periscope is down then the submarine wouldn't be spotted, however with the periscope up it would be visible by the wake left.
A Close enough bomb (250lb direct hit, 500lb close hit) would be enough to sink a submarine, basically using the same formula as the ground vehicle, a close enough hit can either damage the torpedo tubes or the engine.
I certainly agree on adding submarines to Port defense, as it is a single vehicle hanger is just not enough defense at a port.
One suicidal bomb**** and there is no way to defend the port, perhaps adding a second VH would require more precision.