Bad grounds can do all sort of screwy stuff, but since they usually manifest in a major way (ie...completely dead ski) most likely its inside of the switch housing.
Start by
downloading the manual here See Pages 7-6 & 7-9 get a meter and test continuity on the switch. If it passes then your problem is likely in the CDI - if it fails (my guess is it will) then keep reading
Open it up and you'll see there are the two buttons which are only single leg contacts that complete a circuit when depressed, and a third part- a rectangular little housing/box with a plunger at one end that is actuated by the lanyard raising or lowering the spring loaded perch it clips into. Both start and stop switches actually run through this lanyard box and most likely your problem is inside of there. Its a nifty little contraption, offset spring loaded blades sliding over contact points. Anyhow i had a ski where the 'start' portion of the switch died, i was able to bypass that little box by snipping and soldering the wires together so the starter will always turn regardless of lanyard position (thats even how our Octane came from factory. I believe you could do the same for the stop button but would sacrifice lanyard function completely (not a great idea). Open it up and you may find a rotten spring in yours or crusty contacts that can be cleaned up easily. reinstall with dielectric grease and you may buy some time.
Otherwise call Jet Ski Solutions, they rebuild em cheaper than buying new, or troll the for sale section/post a WTB-parts for used
