Indeed a marine centrifugal can only pump to about 10 psi head pressure but needs the proper 12vdc motor to permit continuous operation.

I temporarily shut OFF the pressure water pump, temporarily connect the centrifugal pump into the hot water side of the pressure system .... the centrifugal constantly turns over about 1-2 gpm through the water heater (with engine coil) and continuously pumps (circulates) back to the tank. All my pumps are with 'quick disconnects'. Can be used when connected to 120v or when engine is running (preferred).

Caution: a UHMWPE tank will begin to 'soften' at greater than 165° water temp., UHMWPE will begin to 'flow' at 180°F

Heat Load (100 gal tank): 100 gallons = 834lb; 165°-60°=105°∆T; 105°∆T x 834 lb = ~ 90,000 BTU storage (potential)