A few things come to mind,
your engine turns a hollow shaft that the jet pump shaft goes inside. This is the spline inside of the hollow tube attached to your engine that goes to the rear , if this tube is stripped it will slip.
with the engine off you should not be able to turn the jetpump shaft from underneath.
Also is the engine coupler that goes to the driveshaft intact? these are made from aluminum and can fail/crack ect
The other thing that you might want to check is the jet pump impeller to housing clearance, is it in specs?If the impeller and housing are worn (from sand and other debri) the clearance might be excessive and cause slow accelleration
Johnjon
October 2006