This is what an underwater atomic explosion looks like

Wahoo and Umbrella were code names for two underwater nuclear tests conducted in 1958.
Wahoo was detonated on May 16, 1958 and Umbrella was detonated on June 8, 1958. Pat Bradley, the cameraman who photographed these events recounts his first hand experience of seeing these tests and being on the island as the explosion took place.

You have to see the footage starting at 1:46, some pretty big surfing waves.