Ah Bowfishing.... I just got back into bowfishing this year, and i got to say i reach for my bow now more then my rods in the summer.
Like 7danny said know your regs first, where i live they encourage us to shoot all the carps, gar and especially Snake heads we can handle. I would get an old bow to start, im using my old Browning bow i used to hunt with when i was a teenager, no peep or loop or any of that crap, get good with your fingers. If you use a sight sight it in low.. I mean put a paper plate in the yard and shoot at it, and make sure the arrow is hitting a good 8 inches below the plate, Thats on in the bowfishing world, water refraction is something you got to get used too....
AMS has a bunch of crap for bowfishing, its what they do. I highly recommend spending the money on one of their reels, and get a good rest too, you can use any rest you want, but the ams rest or the whisker bisquit bowfishing rest are great. Any fiberglass arrow will work, and i love Interloc heads(3 barb grapple point), not a fan of others i have tried.
http://www.amsbowfishing.com/
http://www.innerloc.com/Pages/prod_H2O.htm
And lastly..... Have a blast!