I second George's response. a Dayton 110volt shallow well pump from Grainger does a great job, and the impeller won't burn up if run dry. Not so with the GM, particularly with guests on board taking princess showers. The dayton comes with a pressure switch, and it is great to have both. For whatever reason, we have had more failures in this system than i would have ever guessed.
I met some liveaboards years ago on a Hatt in the Bahamas, and they had 3 Dayton 110volt pumps. 2 installed and one in the box. That was their insurance against no running water. inverter and 2 gens probably helped too. There are cheaper ones out there, and perhaps they work as well.