"This is interesting. Never heard them talk about underwater...

"This is interesting. Never heard them talk about underwater communications."

Me neither, but makes sense when you think about it.

The best light transmission through water occurs at blue and violet wavelengths. Anyone who has ever done any skin diving, or scuba diving, will know that warmer colours rapidly fade as you descend. At 20m depth very little remains apart from blue and blue-green, with red objects already appearing black.

Underwater communication currently uses sound and long wavelength radio (VLF, ULF & ELF). Both suffer from very limited bandwidth, and can be easily eavesdropped by others. Information can obviously be protected by encryption, but only at the cost of using up most of the valuable bandwidth in the process.

By contrast, line-of-sight laser communication will allow much more information to be transmitted (potentially a sizable fraction of fibre-optic data speeds), and will also be hard for others to detect. This could be useful in situations where acoustic comms are currently used, such as diver to diver and ship to diver communication. It could also be very good for ship-to-submarine communication (say within a carrier fleet). The sub would need to come within range, but once in position broadband communication would be possible. A further application could be untethered ROVs.

The following graph is sourced from Wikipedia: