Use a set of jumper gables and jump from the positive input lug on the solenoid to the positive lug on the starter, If the starter engages, you have a continuity problem that is either at the solenoid internal contacts, or in the wire connections on the solenoid, or the starter. A bad connection doesn’t necessarily look bad to the eye, clean all connections anyway. A good coat of liquid tape completely covering all connections will help prevent future problems. That goes for all electrical connections on your boat. An ounce of prevention is a good way to a good day of fishing. It’s a long ride back to the ramp with a trolling motor.
Never look down on someone unless you're helping them up.