Inflate the boat and spray it with soapy water. This will let you see the hole.(Even if its a pinhole leak) If you can remove the hardshell floor do so and spray the complete boat. When found, circle the hole with a marker pen and repair. Depending on what material the boat is made of dictates your repair process. Is it a PVC construction or a rubber compound such as Hypalon or the like. A good website is
www.nrsweb.com This is Northwest River Supplies in Idaho .They are a major whitewater supply company and have some instructional videos on how to repair inflatables. If still stuck...pm me and i'll see if i can help you out more.