but I think having one that can operate from internal batteries or the boat's 12v is best.

However, the GPS system is operated by the military, and as evidenced by the aberrations following 9/11 and various notices of "testing" since then -- the DOD my introduce deliberate errors or shut it off without notice -- or a threat might jam it. So if you want redundancy it's best to have paper charts and compass and bearing compass (coastal) or sextant and sight reduction tables offshore.