@Imperious:
Why would the magic number be two rounds rather than one?
The basic OOB system rules gives to the attacker, in fact, an unlimited number to destroy as many transports as there is in a sea zone.
There is some cases, when transports are on an amphibious assault and the defender choose to scramble up to 3 fighters, which give 1 round of defending fire before allowing retreat and, in this case, specify that 1 transport is sunk for each fighter scrambled.
In the previous case, the basic principle that 1 combat unit destroys 1 unit/ round is granted.
Only exception: AAA.
So we got 2 extremes situations: one based on the OOB rule about defenseless transport and, on this last case, 1 round of attack on defenceless transport on offence.
Giving one additional round of attack, make this escape effect on transport (which cannot usually escape), less effective than a sub submerge ability (once the destroyer blocker is gone).
Keeping the principle: 1 combat unit destroys 1 unit/round, allowing a second round, makes the attack, at the end, a 1 combat unit destroys 2 defenceless transports.
It is a middle term between (1 kill/unit and 3kill/units).
Only one additional round is easier to remember and keeps track (than 3, 4 or more).
And it gives each battleship (the best gunships of all) a real advantage over other units.
Each BB can sink 4 transports before they escape.
Allowing more rounds of destruction, will mean, on average, the same thing as the OOB rules: no transport will survive.
EDIT: A single round of fire before escape is almost giving transports a better escaping capacity than the subs “submerge capacity” which needs to kill all enemy’s DDs before withdrawing of battle. Hence, giving 2 rounds of enemy’s fire is a clear difference between submerging after first round and escaping on the ocean surface.