High voltage over 3500 rpms

[Carl]

A problem I experienced last season -

When running around 3500 rpms or higher, my garmin fishfinder (160 red) would flash a message stating the voltage was too high and within a few seconds would shut itself off. It is the only electroninc equipment I have onboard so I don't have anything else to compare it to. My volt meter would be reading around 18 volts in these situations. Is this a problem? If so, how do I diagnose and fix it. It's almost warm enough here to start working on the boat. I have a 1988 Mercury / Mariner 150.

Thanks for the help