You know how water is blue, right? The reason water is blue is because it absorbs the red part of the solar spectrum. This, unfortunately, is a big chunk of the energy that solar cells like to suck down, which is why we don't have solar powered submarines, but the Naval Research Lab has designed a new type of cell that does work under da sea.
The key to harvesting energy from light underwater is to focus on the light that doesn't get sucked up by all the H2O in between you and the surface. This means designing a solar cell that works most efficiently with the light in the blue and green parts of the spectrum. Instead of using crystalline silicon, the NRL went with gallium indium phosphide, which is most efficient in wavelengths between 400 and 700 nanometers, ideal for underwater use. Because of the narrow spectrum, the cells can be very efficient, and even though the overall intensity of light is much lower underwater, the cells can eke out enough juice to power sensors 30 feet down.
The NRL says that this could be a key development of long-duration autonomous underwater systems and sensor platforms, and we wholeheartedly agree: clearly, solar powered submarines are the future.