An anchor buried in the mud and the ground plate on your boat may make a
great electricity source. Run an insulated wire from the anchor and one from
the ground plate to a simple dc-dc converter and you can charge your
batteries:

http://www.nature.com/cgi-taf/DynaPa...20/n8/abs/nbt7
16.html&dynoptions=doi1061334452

Jax