You can look here for exploded parts diagrams:
Xtreme Powersports' Sea Doo PWC Parts Finder
specifically, here:
Xtreme Powersports' 1996 Sea Doo GTI Parts List
and here:
Xtreme Powersports' 1996 Sea Doo GTI Parts List
The engine output shaft to the impeller is either working, or broken. I seriously doubt there is any inbetween state of the drive shaft.
If the driveshaft is turning (part #11) then it should be turning the impeller (part 1 on second drawing) If the impeller is turning, but not creating thrust, the most likely reason is because:
The impeller isn't getting water throught the intake grate (part 32)
The impeller itself is obstructed / tangled with debris
The impeller is physically damaged
The wear ring clearance is too large (part 8)
Other places to look - impeller housing, impeller shaft, etc.