I make jokes about bikini snow, but it's important to realize that stuff didn't just fall on an empty desert. The human and environmental toll taken by the testing of atomic weapons in the pacific is a topic that will be covered soon in the series with Dr. Kathryn Higley.
Phil explained radioactive isotopes and biological analogs in a series of DMs I'll post here. I had read some conflicting information about whether biology would replace sodium or potassium with Cesium-137, and asked him to clear it up for me.
They'll use potassium too rather than sodium. Our specific concern would be the radioactive K-40, which you will totally make in an oceanic detonation. K-40 is also naturally occurring, so you have to separate the bomb blast signal from the primordial radioactivity of planet earth itself. It has a half-life on the order of 1.2 billion years so its specific activity (radioactivity per unit mass) is kinda low. Cs-137 on the other hand is short lived, so high specific activity, and a fission product. If you see Cs-137, someone set off a nuclear weapon or your local nuclear reactor is leaking.
Sr-90, another high specific activity fission product, is a chemical analog of calcium. You want to know if someone lived in a place with weapons fallout or bad reactor fuel, you look at their bones and teeth. In the water, you'd look at corals and mollusks for how much they substituted in their CaCO3 shell for SrCO3.
This is how you can also guess how damaging something will be.
Sodium, Potassium, and Cesium are all chemical analogs. Take a peek at the periodic table of the elements and you'll see they're all in the same column, AKA the alkali metals. Compounds they form with the column with Fluorine and Chlorine in it, AKA the halogen gases, are commonly referred to as salts. Some of them are fairly odd salts and HOO NELLY do you not want to substitute NaF for your NaCl table salt in terms of what the fluorine will do in your body.