> Similar for heavy metals. These dissolve into salt water, and were already present in solution since the dawn of time. We won't change those numbers significantly. (again, assuming you're not dumping it just off of land like the Japanese did in the 50s)

This is not true. Due to biomagnification, there is already significant contamination of fish stocks with (methyl)mercury:

https://en.wikipedia.org/wiki/Mercury_in_fish