Seaweeds are prominently responsible for leeching and removing heavy metals and toxins in the body.
Kelp, dulse, blackwradder, all brown, red, and green algaes "sea vegetables", are extremely nutritious, mineral dense, and contains the highest source of Iodine in any food. If you find wild crafted seaweeds, that are sourced from clean parts of the world, you will not find the toxins and heavy metals that you are referring to. It depends on where the seaweed was found. The pros outweigh the cons, and there is so much evidence leading to the fact that edible seaweeds can kill cancerous tumors, among other things.
You can denature seaweed and say it's toxic, however you are not even giving it a chance. I've started taking high amounts of kelp powder, 1-2 tbs. and I've recognized and even feel the difference.
Seaweeds, depending on where it is sourced, if you buy from reputable source, or you can do your research and find out where the seaweed was harvested, that would be the only problem you would have to worry about. Sure picking seaweed next to a sewage dumping ground is not healthy, however using that as an excuse to not take seaweed, and acting like all seaweed is impure, and toxic is actually inaccurate information.
In my opinion, seaweed is a superfood, and our bodies are made up predominantly of water, eating the seaweed also supports our body with Sea Salt , the trace minerals, and it's something our body actually utilizes and is necessary for health.