I haven't used one, but there are xbow reelmounts available.
There is a guy near my home that goes bowfishing with his horizontal bow, but I haven't met up with him yet. When I do, I'll give him your screename so he can post here, or contact you to trade tips.

For about $20, you can get an AMS Retriever mount for an xbow.
http://www.amsbowfishing.com/product_info.php?products_id=241

One thought about the xbows is that they are different than a standard bow with respect to safety. I would imagine that you and most others who would want to use one are experienced shooters, but it's still worth mentioning that you would need to be extra 'muzzle' conscious when it is cocked, just like with any firearm. Pretty easy to turn the wrong way on the deck of a moving boat while tracking a running fish, trying not to knock into your boatmates, and dodging branches.

Post pics when you get your's assembled.
Good luck.