Excellent practical advice. 4 rechargeable cells will give 4.8 volts, but should be perfectly OK. In a small boat, 4 of AAA is possibly better, maybe 5 to give 6 volts. If going the LiPo route, consider 1S plus a step up voltage regulator. Nowadays cheap enough and light enough to be worth considering. They provide a regulated 5 volts from the single cell 3.7.