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The problem is one of a couple things.
You either have weeds, or some sort of debris in the pump blocking the flow of water and it's cavitating or the wear ring and impeller are damaged.Usually the wear ring will be the first to go bad because it's made of reinforced plastic.
You will need to look up through the intake grate with a flash light to make sure you don't have something in there.
And if that's clear you have to remove the jet pump from the back of the ski to inspect the wear ring and impeller.
You should have no more than .030 clearance between the impeller and the wear ring.
If it's grooved or torn then it's junk.
You have to cut the old one out and press the new one in. It's usually something most folks have a shop do because they don't have a press or a saw to cut the old ring out without damaging the pump body.
On the bright side a wear ring can be bought for around $50 so it's not too expensive for parts and labor is usually 1 hour.