Hi!
1) I believe you can, but it'll take you a lot of time, a lot.
I sunk some small fishing boats and sampans with the 40. Mm aa gun.
2) no you can't, you can blow up some of the other crew on the ship with the deck gun but not the ones at the aa guns stations or the deck gun stations
3)it depends, the twin 20. Has a higher rate of fire than the single one, it does the same damage i believe but since you have two barrels if both the bullets hit it'll do twice the damage
Of course, since you fire two bullets per shot you' re gonna run out of ammos faster
Hope it was clear enough, if you have any questions let me know
