AA fire (both on land and at sea) happens at the beginning of every combat round that there are planes attacking ground units or naval units. So Naval AA would get to shoot at planes every turn (prior to planes attacking ships and ships shooting at ships).
Note: Only planes assigned as bombers get shot at by AA. Planes shooting at other planes on the Air-to-Air battleboard don't get shot at - just the planes that are assigned as bombers and are attacking ships (or ground units in a land battle).
Does this help?
Mark