By testing the solenoid I'm going to assume your boat is not turning over to start? If the boat is not turning over when starting and you suspect the solenoid, take a screwdriver with a rubber or plastic handle (to insulate yourself) and make contact to both posts of the solenoid. Be careful, by doing this you are activating an electrical circuit. You only want to do it for a second and be ready to break away contact with the solenoid when/if the engine starts to turn over.