That solenoid looks like a 100% duty rated unit which Id expect to see on a larger air compressor and NOT a 12 VDC fresh water pump. They are easy to test to see if the plunger may be stuck in the closed position. They are normally OPEN (big terminal to other big terminal) but if 12 VDC is applied to the small terminal it closes so the pump runs. The way it works is if the air pressure is below X PSI the solenoid closes (12 VDC to small terminal) but once PSI raises to the cut off pressure it opens and the pump stops running. If the pump continues to run there may be a leak or the pressure switch is bad and Id suspect either of those before the solenoid is stuck in the closed on position.
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