Have Boat Questions? Ask a Boat Repair Expert.
Troubleshooting the electrical is not easy. However what you are experiencing usually is caused by corrosion on a connection and most of the time it is the ground for the engine. Physically take each connector apart, make sure the terminals are clean, you can use a small dab of dielectric grease on the terminals. Take your battery cables off, clean the terminal ends and the mounting locations/block and securely tighten them. You can use some liquid electric tape over the connections on the block and relays. This will keep the corrosion at bay.
So I think corroded, loose connections are your problem.
Try this and post back your results.