The number of undersea tests is quite low and the bomb have not been that large. Water can spread out the radioactive element produce just like if you do it in the air.
The number of underwater nuclear tests is a total of between 1946 and 1962 and the larges bomb was only 30kt
https://en.wikipedia.org/wiki/Underwater_explosion#List_of_underwater_nuclear_tests
Atmospheric tests result in the fallout spreading out over land and it can be a problem. It was recognized quite early on and as a result, the https://en.wikipedia.org/wiki/Partial_Nuclear_Test_Ban_Treaty was singe in 1963 that ban all these that is not underground tests. There was a total of 269 nuclear tests in the atmosphere before the ban. All of them were not large bombs, many of them are tested in the single-digit kilotons range for tecnical tests of different parts of nuclear weapons design.
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