2000 gp1200r will go 5 mph with rpms 8000

Have you checked your impeller, even on the sandiest beach all it takes is 1 pebble to damage it. Its a common problem (all noise and no action) if it was a car you could liken it to clutch slip or wheel spin, when you take the impeller out the damage will be fairly obvious, ding marks on the leading edge and bent areas on the trailing edge. Solution :- Birmingham Screwdriver (Hammer) and cobblers last or tow hitch ball. Gently hammer out the bent bits and smooth the sharp edges off the ding bits (Both of these cause cavitation). This should get you back on the water within the hour. If this sounds a bit too much to have a go at then seek professional help. There are plenty of companies out there who can do the job, it's just a question of cost and time. Give it a go, you never know it might work.