Check the ground-going to the engine by the starter coming from the battery. Have you run the seadoo yet this year...Its possible the starter or solenoid is bad, or worse the engine could be seized up. Go to the solenoid and use a 14 guage insulated wire to jump the solenoid to see if the starter rolls over. If so the solenoid is bad. If it still clunks, on the rear of the engine is a black boot with 2 clamps on it. Loosen up the clamps and push the boot back. Remove the spark plugs and see if the shaft under the boot turns counter clockwise. Use a pipe wrench if you need to being carefully not to hurt your hands or damage the seadoo. If it won't turn over it might be seized. If it turns over remove the starter, as it must be bad.
I hope this helps you.
Karl