Baker was a 23-kiloton nuclear weapon that was detonated underwater at Bikini Atoll in 1946. The goal was to see what would happen to Navy boats if they were in the region where a nuclear bomb went off. The boats you see in this photo were unmanned, but there were sailors relatively close by, taking these shots. There's evidence that they weren't properly protected against fallout, and later used contaminated water to drink and bathe in. (Also, as a fictional side effect, Bikini Atoll nuclear tests like Baker might have been responsible for the creation of Spongebob Squarepants.)
My Modern Met has compiled several photographs and video that give you an up-close, mind-boggling view of the explosion — including the massive column of water that shot into the mushroom cloud and the 2-mile-high tidal wave that followed.