There have been a few requests from scenario writers for a defensive only AI that does not attack. This is the best I can get. The ships may occasionally be aggressive, but I've found they won't attack even a nearby target unless you give them an attack-now command. However, I limited the ship attacking to when the town is under attack. This AI will very vigorously defend its town, so be careful not to trespass unless you intend to spur hostilities.