A Garmin 45, a Magellan 2000XL, etc. I pick them up for $20 or $30 at various flea markets. They still work fine and will serve as back-up if we lose our electronics and in addition to internal batteries the Magellan has a 12v cable. The smaller ones fit well in the ditch bag or in the waterproof bag of stuff (handheld 2 meter radio, mini binoculars, flashlight depthsounder, etc.) that we take when we go off gunkholing in the dinghy or the skiff.