If you sank it before containment was breached nothing would escape into the atmosphere and therefore there would be no fallout. There would be leakage in the

If you sank it before containment was breached nothing would escape into the atmosphere and therefore there would be no fallout. There would be leakage in the ocean itself but its effect is more localized due to water attenuating the radiation. While research is pretty scant, there have been containment failures at sea. Currently 8 submarines nuclear subs have sank. In general their effects have been much less than Chernobyl the only land equivalent.