The ocean is clearly a better option. First, water itself acts as an absorbent of the emitted radiation. So the only living things which take up the radiation

The ocean is clearly a better option. First, water itself acts as an absorbent of the emitted radiation. So the only living things which take up the radiation are those which consume the water. Second, unlike heavy metals, the uptake of heavy water isn't especially driven by biology. They don't soak up and concentrate the radiation. Third, there is a lot more water in the ocean than the atmosphere. In the ocean it will dilute to nearly unmeasurable levels. In the atmosphere it will fall back down as rain and concentrate in our water supply, irrigation, and crops.