You know, I can't say that I've ever seen or heard of anyone bowfishing with a crossbow.

I think a cross bow is really too heavy, too slow (for repeat shots) and has too much poundage for bowfishing. (heavy poundage at short ranges will have you fighting with the bottom of the river or lake more than you'll be fighting fish, and you'll occasionally lose that fight and lose arrows)

The ideal bow fishing bow would be a light, short recurve about 40-45lbs. (in my not-so-humble-opinion)
You can pick up a used recurve for relatively little money. (look in the classifieds here or on ebay) A simple cheapo compound will work too. (that's what I'm currently using)

A basic drum reel and some fish arrows shouldn't cost more than about $40-50 to convert your recurve into a bowfishing rig.
A retriever reel or closed face reel would be a bit more, but still not too expensive.