Piscina is a swimming pool/a pond/a fish pond/a water tank/a public bath/a private bath/a place to wash your clothes;
= all kind of bodies of water
Swimming in a pond is quite possible. If you make an internet search, you'll see than even nowadays, a lot of people in the world do that.
I'm pretty sure than Romans did the same in natural ponds. Why couldn't they? We tend to project a lot of modern cultural reluctances on them.