Didn't the U.S. do exactly that explode a hydrogen bomb underwater near the Bikini Atoll in the Marshal Islands? Seems to me everything was still there afterwards.
The Castle Bravo Test left a one mile crater.The Shrimp device tested in Bravo was a 15 Mt two stage thermonuclear surface burst. This was the first "dry" or solid fuel (lithium deuteride fueled) H-Bomb tested by the U.S., and the first solid fuel Teller-Ulam device ever tested. It was the largest bomb ever tested by the U.S. although this was by accident. The yield of Bravo dramatically exceeded predictions, being about 2.5 times higher than the best guess and almost double the estimated maximum possible yield (6 Mt predicted, estimated yield range 4-8 Mt).
An hour after the shot Navy ships 30 miles south of Bikini found themselves being dusted with fallout with deck radiation levels rising to 5 Rads per hour. navy personnel were forced to retreat below decks and the ships retreated farther from the atoll.
As the fallout drifted east U.S. evacuation efforts lagged behind the plume. At Rongerik, 133 nm from ground zero, 28 U.S. personnel manning a weather station were evacuated on 2 March but not before receiving significant exposures. Evacuations of the 154 Marshallese Islanders only 100 nm from the shot did not begin until the morning of 3 March. Radiation safety personnel computed that the islanders received a whole-body radiation doses of 175 rad on Rongelap, 69 rad on Ailinginae, and 14 rad on Utirik.The Japanese fishing vessel Daigo Fukuryu Maru (Fifth Lucky Dragon) was also heavily contaminated, with the 23 crewmen receiving exposures of 300 Rads, one of whom later died - apparently from complications. This incident created an international uproar, and a diplomatic crisis with Japan.The entire Bikini Atoll was contaminated to varying degrees and plans for conducting test operations from the islands, including use of the firing bunker, had to be abandoned. All further Castle tests were controlled by radio link from the USS Estes.
www.youtube.com/watch?v=Ge865CR9pN8