Boat electrical systems can have have a lot of noise in them. I had issues with wild excursions of the main 12V bus when turning on (and even worse, turning off) various pumps and the windlass. The excursions can be really quick- too fast to measure with normal tools, but enough to trigger false positives in Arduino digital inputs, in this case shown with the “switch” being a proximity sensor. The solution was a low pass filter made from a resistor/capacitor combination.
(Use 50V or higher ceramic capacitor) This was recommended to me by a CruisingForum member, and it totally fixed the problem.
The same type of filter also works well for analog signals. The X Engineering Alternator regulator uses one to prepare the “RPM” signal coming from the alternator stator tap, to prevent false positives. I will be including them in just about all future sensing circuits.