I mean, the steps could be a safety device if you fell in since there’s no safety rails around it. Humps could just be sediment.

Again, I’m speculating, but I’ve seen those kinds of basins before.

Speculation but: Children.

If you have something that looks like a pool they’ll go swimming.

The ladder gets them out and the humps give them a place to stand in the middle to keep their heads above the water line. Their arms get tired very quickly.