Photos of an Underwater Nuclear Explosion
Although not strictly "underwater" the photos are of an "underwater explosion". PetaPixel recently posted these photos and a short video of Operation Crossroads, the US's Nuclear Tests on Bikini Atoll in 1946.
The goal of the tests was to see how navy ships would be effected. Judging from the images, this editor would have to say they fared badly. A unique look at the ocean and the destruction we as humans have wreaked on it.