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The most important thing you have to do is a compression test. Go to an auto parts store and buy a compression gauge (they are about $20-$25.)

Here is a good read on buying a used jet ski:

Shop SBT - What to check when buying a used ski

Pull off the wires and do a dry crank. Ground the plug wires on the appropriate terminals on Sea Doo’s and other skis that have them ( ultra 150 and some others). Does the motor crank slowly or strong? If its slow it could be anything from a loose electrical connection to a bad battery to a bad starter.
Ok we are going to assume the ski is good so far. Pull off one of the plugs and thread your compression gauge on there. Pull the throttle all the way open and then crank the ski. Watch to see how high the gauge gets. Stop cranking and check reading. As a rule of thumb you’re looking for about 150 PSI for a two cylinder motor and about 120-130 for a triple cylinder. Anything less than 10% ( I prefer 6% but 10 % is a accepted basic guideline) between the cylinders could mean a new top end is in your future.
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