I have built navies as the Cumans in previous versions. They had the Islamic boat system. Do they not have them in V 3.0? If not, here's how you can enable them. First, you need gnome editor. Pretty sure it can be downloaded somewhere around here...

Anyways, after that, its relatively simple. You need to go to "my downloads", select gnome crusade unit editor, two times. You'll want two editors up at once so you can better make sure you're making the same changes to both time periods for the Cumans(they aren't in late period, as they met an unfortunate demise at the hands of the Mongols ) When you've got two editors up, you'll want to go into the "total war file" under program files. Then, select "MTW XL unitprod early" and "MTW XL unitprod high" respectively. Scroll down until you get to the ship entries. They'll all be grouped together. Figure out what boats you want to give them. Then scroll all the way right until you get to about column 56, which is cultures. Add in PAGAN to the boats you wish to make available to them. Make sure to put a " " around the two/three cultures if it didn't already have them, as they will be needed for multiple cultures. Then scroll back left to column 50, which should read "faction asssociation", and enter FN_CUMANS in alongside the other factions for the boat of your choice. Save. And voila! Your Cumans can now produce ships!

Spoiler Alert, click show to read: 
Hope those instructions weren't too archaic.