Could be a host of stuff!!
filter clog, screen in tank cloged, is the ball staying hard?? Do you have a water seprator?? when did you change it last??
Water in the gas??
Carb rebuild?
Someone here might be able to help better then I?? I no my own boat and I can trouble shot it but with out noing your boat and when you last did all your maintance makes it tuff!
You could try Iboats, they have a lot of great people over there and its free!! They have helped me alot in the past!!
http://www.iboats.com/cgi-bin/ubb/ultimatebb.cgi
Good Luck!!