Taking Ski's advice I checked out Kawasaki's jet skis on line. The smallest has a 1.5 liter, 4 cylinder engine. No way is that going to work with an RIB, it has to weigh 250 lbs.
Neither do I think that the 125 cc powered surfboard will work. Due to the inefficiency of a jet drive you will need more power than the typical 200 cc 9.9 hp outboard if you want to plane. So you will probably need 350 ccs which is typical of 15-20 hp outboards.
A 20 hp Yamaha outboard weighs about 110 lbs, so a jet drive built around the same engine should weigh approximately the same, maybe more.
So are there any 20 hp 350 CC jet skis on the market? In googling this subject I found a 7 hp powered kayak-
Tech Specs — Mokai. The engine and pump weighs 50 lbs and is based on a 211 cc engine. But it sure wouldn't plane a 10' RIB.
David