That pump should work. The Johnson pump in that size might be quieter...
I'm surprised you don't have an accumulator tank -- what boat Mike?
As for the 10 gallons/minute...I don't believe that number. Real world pump output really relates to head pressure you're overcoming (e.g., How high above the tank you're pumping-up or -down) , resistance in the system (bends, joints, etc.) Still
Your hot water heater serves the galley and the head/shower?