maybe you should take it to a mechanic if you cant understand that. Ha. thats fine. the impeller is the propellor and there is a housing around it. it should be a tight fit in the housing which is around 2-7thou which is incriments of a milimeter. Cavatating is air rating the water under the ski which means it has no traction. Like spinning wheels or slipping clutch on a car. If the engine is not dying upon acceleration, and its reving up and not going very fast, its cavatation.
You should take to a mechanic (jetski mechanic). The pump shoe seal would have blown out which means you will need to remove the pump shoe and totally seal it up whilst the pump unit is out, check the liner gap and the impeller for damage. You need to pull it apart to fix it mate. If i were you id just take it to a mech.