Look to the spark plug first. Remove the plug from the cold engine and inspect it closely. A hairline crack could leak on the compression stroke. Also, it may have rattled loose over time, allowing compression gases to escape. If the plug was removed in the past to clean it, perhaps the mechanic was a "hack" and did not set the torque per the manufacturer's specifications. If the plug is loose, torque it down per the jet ski's recommended torque. Mechanics at the Georgia Z Club of the Community College of Baltimore County recommend using a compression gauge to test combustion chamber pressures. You are assured the problem is truly low compression, and not blow-by through the plug if you test with a gauge.
If the rings are worn or broken, they allow compression gases to blow by the piston and into the crankcase, instead of being trapped in the combustion chamber. This condition is so common, its called piston blow-by. The only cure for bad rings is to pull the piston and replace the rings. This is major surgery on an engine, and should be done only by qualified jet ski mechanics.
A warped head, or a blown head gasket results in low compression. What happens is combustion pressure escapes out a small gap between the block and the head. The only way to tell is to remove the head, and have it inspected at a machine shop that has the specialized equipment to measure head warpage. If the head passes inspection, the problem lies elsewhere.
On 4-stroke engines, if the valves are bad, they will stop sealing. The only way to tell, is to pull the head and have a machine shop inspect the valves and seats. The only solution is to obtain new valves, and the shop can grind and fit them to the head. Grinding and fitting has to be done. New valves and the seat have to match. The only way to do this is by grinding them into the seats with valve lapping compound.