What is wrong with your jet ski if the engine will open up all of the way but jet ski will not go very fast on full throttle?

It sounds as if you may have an impeller or driveshaft issue. If the impeller, or the impeller housing is badly worn, the craft will loose power due to excessive tolerance between the two parts. This is very common when used in sandy bottom, shallow water conditions, as the sand picked up by the rotating impeller wears away at both the impeller, and the housing. Also, in the case of little, or no service maintenance, the driveshaft splines may be severely worn, due to no grease, at the coupler or the impeller end, causing slippage.