"Well, a population study comparing women with breast cancer to women without found that consuming a single sheet of nori a day may cut a woman's odds of breast cancer in half." — Dr Greger

"We think it's because seaweed favorably alters estrogen metabolism, likely due to modulating women's gut bacteria. The more seaweed we eat, the less estrogen we have in our system, which may lower breast cancer risk." — Dr Greger

"The breast cancer protection may be because of all the fiber in sea vegetables, or because seaweed may block the enzyme that undermines our body's attempt to flush out excess hormones. Or seaweed may somehow interfere with estrogen binding to estrogen receptors. Whatever the cause, to effectively lower their estrogen levels, Asian women may be able to get away with about one sheet of nori a day, but American women are physically so much larger that it may take closer to two."

As you can see, rice contains more of the toxic inorganic arsenic than seafood, with one exception. Hijiki, an edible seaweed—a hundred times more contaminated than rice, leading some researchers to refer to it as the “so-called edible…seaweed.” Governments have started to agree. “In 2001, the Canadian [government] advised the public not to eat hijiki.” Then, the UK, the rest of Europe, Australia, New Zealand, then China “advised the public not to eat hijiki, and banned imports and sales” of the stuff. Japan, where they actually have a hijiki industry, just advised moderation.

So, I’d recommend to avoid hijiki due to its excess arsenic content, and avoid kelp due to its excess iodine. But all other seaweeds should be fine, as long as you don’t eat them with too much rice.