this is a littel last minute but I just found a cool product on Kickstarter. drag it behind your boat and charge batteries. Hydrobee Harvests Water Energy for USB Power to Go by Burt Hamner — Kickstarter
the HydroBee looks like a great piece of gear for charging camaras on multi day trips. By the way I am in no way ascoicated with this product in any way shape or form and I noramly dont promote products but I thought I would kick this out to the forum. Also if this is your type of thing you better get behind it fast, as of the time I post this there is only 36 hours left on kickstarter before this product fails and goes away forever.