One of the things I recommend is to install a quality battery monitor on the boat. There are times when the house batteries are below the nominal operating voltage of the GPS, particularly when other items are running. My Lowrance has a built-in battery voltage monitor, which is extremely accurate. Keep in mind that if the voltage drops below a certain point the display may come on, but you may not have sufficient voltage to accurately acquire a satellite signal, which is very weak to begin with. Additionally, if your GPS/Plotter has a built-in antenna, which the vast majority do, you must keep the top of the GPS free of dirt, which can impede the signal strength. Same holds true with the GPS position on the boat. Under certain conditions, the sails, boom, stainless rigging, bimini frame, and lots of other items can block the satellite signals. A good example of this is the Spot, which relies on it's ability to send a miniscule signal to an orbiting satellite. It must have a completely unobstructed view of the southwest sky or the signal is blocked.
Lots of think about with these devices, but they still, IMO, are great navigational tools.