PowerGenix is touting their Nickel-zinc batteries... looks like another possibly interesting technology.

The basic properties inherently found in a nickel-zinc (NiZn) battery make it a very attractive power source for a wide variety of DC-powered devices. Its high energy density and high discharge rate capability make it an ideal solution for applications that demand large amounts of power in a small and lightweight package. Cordless power tools, premium UPS systems, electric scooters, specialty military equipment, and high-intensity DC lighting are among the applications in which rechargeable NiZn batteries are best applied.

I got pointed to their site from this discussion of using their technology in electric scooters. We've had bicycles before (even electric bikes), but a scooter is another interesting transportation alternative for your boat, assuming space allows.