Rest assured it doesn't just disappear. Some gets to the ocean, some settles to the bottom of Lake Ontario. The further you can get upstream, the less polution. I agree, there is polution in all waters to some degree. A fish from the north shore of Huron, has not seen the polution floating downstream from Windsor, Detroit, Buffalo, Sarnia, Hamilton, Toronto etc.
Think about the Don River, at its many headwater sources, it is fairly clean. After getting through Toronto and with all the drains and sewers emptying into it, it's not so clean at the mouth on lakeshore.
If a piece of human crap (commonly seen in Lake Ontario) does make it to the ocean, at least it's worn down to bait size and gobbled up by a fish.