Just call it a pond
Growing up we had a pond on my relatives farm that we swam in. Every winter we'd drain it and muck it out with the cat*. In the summer we'd swim with the bullfrogs and fish among the lily pads. I can't remember the last time I was in an actual swimming pool. Most of my swimming now is in rivers as an addendum to my whitewater rafting.
*Caterpillar Tractor