The difference is that those tests were done on or near land, the fallout settled either on land or in shallow water, close to marine life.
The explosions which happened in deep water (Wigwam in 1955 for instance) had practically zero lasting effects beyond radioactive steam entering the atmosphere.
Some Marshall Islanders would like a word[0]
[0] https://www.theguardian.com/world/2014/mar/02/bikini-atoll-n...