I found that blue light with a wavelength of 400-480 nm can penetrate hundreds of meters and still be measurable. Of course, there should be many difficulties in actually realizing a blue light based system because of refraction and the effects of the surfaceAn illustration from a publication showing what China expects the future of underwater warfare to look like. It involves extensive use of unmanned underwater vehicles (UUVs) as force multipliers in both surveillance and attack roles, while manned submarines staying further behind and acting more as a command node that remotely directs UUVs. Interestingly, underwater weapon magazines (holding torpedos, mines, and missiles) are pre-installed at strategic locations and can be remotely activated and directed to ambush nearby targets. Also of interest is the laser communication system between UAVs and submerged submarines.