I grew up next to the river and have seen/smelt some of the stuff that has ended up in the waterway over the years. I know of a radiator repair business that used to be in Rivervale that would regularly flush waste into the river not far from the ski area

. If I were you I wouldn't risk swimming in the Swan. Nowadays there are frequent algal blooms that are pretty toxic in parts. Despite what others have said to the contrary, I definitely wouldn't eat any fish nor prawns etc caught in the Swan.
If your keen to swim I would heat to an Aquatic centre and use the pools rather than risk getting sick from the Swan.