Lowrance Ifinders are good handheld units, but they do not do street routing. You can get street maps, but it will not give directions. Many of the Garmin Nuvi models will accept the Lakemaster maps. If your planning on using it on the water, I’m not sure how “water resistant” they are. Otherwise, Garmin makes several true handheld GPS devices that would be suited to outdoor use and offer street routing. One that comes to mind is the Garmin GPSMAP 76 line. Biggest downfall with these is the smaller screens. Obviously, the best approach would be to own a dedicated mobile device in the car and a dedicated marine device for the boat, but the MAP 76 would be a good compromise.