There are specific paints available for tubes, here's a link for something you could try.
SP54 Hypalon Antifoul grey for inflatable boats, dinghy and RIBs
In my area, most boats are kept on swinging moorings in tidal creeks. A lot of rib owners just use the same antifoul paint on both their hulls and their tubes. Not sure how good that is for the boats in the long run but people don't seem to have any issues with it. Personally we don't bother antifouling the tubes, we just scrub them every two weeks. It only takes 20 minutes or so and it keeps the tubes looking nice.