An invention that I'm edging towards involves a robotic submarine that can run for many months using it's surroundings for a fuel source. Maybe it can eat algea, but algea doesn't seem dense enough. Then I find this, it says there's hydrogen in sea water, enough to power something?
http://www.washingtontimes.com/news/2012/apr/4/scientists-build-robot-jellyfish-fueled-sea-water/