A nuclear scientist could have told them that. Radioactive particles still have their chemical properties. As a rule of thumb, the lighter they are, the more able they are to travel, because the heavier ones sink in the air or water.
Lighter isotopes also generally have shorter half lives, so they can’t travel too far before they are no longer radioactive. The most dangerous isotopes are the medium sized ones, especially if of elements that are easily absorbed by plants and animals. But in addition to everything else, they tend to disperse.
You likely have some of these in your body, but too few to even notice.
In any event, the Pacific Ocean has about 180 million cubic miles of water. However much medium sized radioactive particles released from Japan would be spread out in this.
US has unintentionally sunk two nuclear sub reactors in the ocean. Soviets sunk a lot more between accidents and just dumping old naval reactors in the arctic ocean. Doesn’t get near as much attention as Chernobyl or Fukushima, but certainly is significant.