Bostik 2402 is what most boat builders use and should be available from most good Chandlers. If it's a coner of a seam or patch then I've used some Polymarine Inflatable Glue (Contact adhesive), mines in a tube. I'm sure that there would be quite a few different products on the shelf for repairing Tubes on RIBs, just make sure it says Hypalon on the side as some are PVC specific and are not suitable.
Looks Slow but is Fast
Member of the ebay Blue RIB cover club.