Amazing! It uses water as fuel!
A platinum coated robotic jellyfish uses chemical reactions between oxygen and hydrogen that happen in water to power itself.
It still needs some sort of catalyst to split the water into its separate components (oxygen and hydrogen) but they might just have a solar powered fueling station floating on the water's surface to solve that problem.
Check out the above video to see it swimming around, eerily similar to how a real jellyfish does.