92 - C115 Yamaha. One year ago temp alarm sounded and water press gauge showed 5 psig. Mechanic said the impeller had "spun" on the shaft. First use after new water pump, I noticed initially good water pressure then a severe drop in pressure upon first throttle up. Mechanic determined the water pressure regulator (poppet) had stuck in the open position. He blamed higher water pressure from the new water pump and corrosion. After new poppet valve and spring, and two new thermostats for good measure, no more temp alarms and water pressure at 17 - 18 psig at cruising speed. Last week the pressure was running 12 psig at 3800 rpms and would drop even lower when the engine speed increased. Running very long under 10 psig caused the high temp alarm to sound. When I would stop and let the engine idle down for a few seconds, I could put in gear and throttle up and the pressure would go up to 18 - 20 psig, but would began dropping again after a few seconds. If I did not put it in gear and increased rpms, the pressure would hold near 20 psig, but as soon as I put in gear and throttled up, the pressure would start dropping. No obstructions can be seen at water intakes and the pee stream has a seemingly good flow.