A couple problems that I see are:
It is really not a good idea to run the engine of a jet boat out of water without the drive line disconnected.
Radial clearance between wear ring and impeller (new) is .025 of an inch. A surprising amount of road grime finds its way into this gap. Running it on the trailer causes the wear ring and impeller to abrade, opening up this clearance, and degrading pump performance.
Another issue is the packings. The packings are actually lubricated leakage. Water pressure in the suction housing leaks past the packings, cooling and lubricating them. This is just not possible on a garden hose. If you are running the engine while on the launch ramp, you will need to need to have the pump deep enough in the water to completely submerge the impeller and pickings. This however presents another problem. The pump will suck debris off of the ramp, again abrading the impeller clearance.
Assuming you have a standard A cut impeller, there is only .125 of an inch clearance between the impeller shroud and the bowl. This presents a substantial restriction, between the impeller discharge and the cooling port in the suction housing. You need to have some decent pump flow (RPM) to provide adequate cooling.
GT Jets has a good recipe with a 3/4 pipe gate valve. Restriction increases as flow increases, providing sufficient cooling flow at idle while not blowing gaskets and freeze plugs at peak power. I think the best results will be tuning while underway.
