Generally I try and be strong at sea where the AI isn't. Playing the Moors I'm island hopping down the Mediterranean creating strong points as bait for the AI to attack. I then use my fleet to sink his ships and men, draining his resources. The upkeep of the ships is expensive but so far the war of attrition has been in my favor. The downside has been the AI dominance of the channel. I've had to recruit the odd mercenary fleet and dash across the straits in my pursuit of the English whilst they cruise across every turn to repeatedly besiege Caen. I also try to build a couple of advanced shipbuilders in the west ready for the 'World is Round' event.