Yamaha OX66 250 HP issues - loss of water pressure/cavitation

The boat is a 30' Scarab with twin 250 hp Yamaha OX66's. Over the winter we took the boat in to the mechanic to get annual maintenance. Turned out that the starboard motor lower unit was on its last leg and needed replacing. We also had water pumps replaced in both motors.

Now we are having an issue with the starboard motor cavitating and losing water pressure (eventually getting hot and alarm sounding) when running above 4500 rpms or around sharp curves. This happens whether the motor is all the way down or trimmed up in its running position. It seems to me that once the water pump loses prime, it doesn't recover. Ideally, I would take the boat back to the mechanic to get it looked at, but its down at the camp and its not the easiest task. Looking for some suggestions on what the issue might be and if it is something I can try and tackle myself.