[eesmith]

"safe" means "safe", which should include:

Will you get ill from the water? (As you mentioned.)

Is it separated from watercraft traffic? (I've been to swimming areas with buoys to mark off where motorboats are not allowed.)

Is there enough safety infrastructure? (For faster rivers, people can get pulled downstream too far. Is there a rope at the end of the bathing area to help people catch themselves? If the river has steep banks or walls, are there ladders for people to get out? Are there life rings and poles so people on shore can help those who get into trouble?) While this could lead to drowning, other possibilities can include hypothermia, or if someone managed to get onto small island but is exhausted and can't get off.

Are there appropriate cautions posted or even closures for things like (this shows my beach background) unusually strong rip tides or jellyfish swarms?

Is the area physically appropriate for swimmers? (I'm old enough to live through the pull-tab era, where people would toss the sharp-edged tabs into the sand, then someone else would step on it and cut their foot, eg, https://pulltabarchaeology.com/archaeology/ . Or read about the "syringe tide" at https://en.wikipedia.org/wiki/Syringe_tide .)