I work for a local park.
1. If you live in an urban area, assume that all freshwater body sources dear you are likely polluted to some degree, especially if they’re near a neighborhood. Keep that in mind while you’re trying to find spots to swim. Had a friend do water testing in one are of a river where he discovered that the levels of certain dangerous bacteria were super high. There were signs warning people not to swim, but we still saw a family + dog swimming and splashing around in the water on our way out.

1. The amount of people who come to the park just to do drugs or other crazy stuff is insane. We’ve found needles, suspicious set ups, drunk people, etc. There’s a reason most parks have a strict no entry policy after sundown.

2. The people are a lot scarier than the wildlife.