I think the big battery drain is actually the screen, more than the GPS (when in active mapping mode). Same thing happens when my kids play Angry Birds. In 2 hours my phone could be down to 20% battery.
However, turning off GPS by default can extend the battery life on a daily basis.
In terms of waterproofness, maybe that's another product idea. Some sort of integrated waterproof housing with a 3200mah battery + gps module + power cable. At the right price I'd buy one.
hey craftycoder, how hard is it to tap into the voice capabilities? It'd be great if you app could support simple voice commands via blutooth.