Well, consider for a second, what fallout is. Debris pulled up in the explosion, and the bomb itself, and stuff in the immediate vicinity of the explosion, made radioactive by the high neutron and gamma flux.
A “water shot” would primarily irradiate water, transmuting the hydrogen and oxygen into radioisotopes.
The radioisotopes of Oxygen have half-lives between 2 and 122 seconds, so they would be violently radioactive immediately, but by the time the water made it out to the sea, it would barely be radioactive at all: 20 minutes would be 10 half-lives of the longest-lived isotope, or roughly one tenth of a percent of the original radiation.
Hydrogen would be transmuted to Tritium, which is a Beta emitter, and thus not really a problem. . .
Salgak,
Well, that was quite an intelligent response.
I hereby retract my sarcastic “Sharpshooter” allegation and I learned something.
Why worry about radiation at all. A Bunker buster at the base of the dam would have the same affect.
10 half-lives of the longest-lived isotope, or roughly one tenth of a percent of the original radiation.
...goes as 1/2,1/4,1/8 so...
10 half lives = 1/210 = 1/1024 or a bit more than a thousandth.