I'm not sure of the exact naval units the Moors get, but for most factions you get a basic ship and a more advanced ship until around turn 105 - the advent of gunpowder. You then get gunboats and eventually, when the world is discovered as round around turn 150 (although I've never got that far) you get ocean going giant ships.

So you just need to wait for those events to occur before you get the opportunity to recruit the larger vessels.