Any pool would have to be above ground to offer swimming, with the height probably fairly close to the bottom of a small house's roof to get the required depth. They would also be fairly small in diameter since they would have to fit in the yard of a small so you wouldn't get a good swimming experience. It would be more like a place to trend water.

Every housing district has an area for swimming so better to use those when you're in that mood.

New ponds to choose from in different styles than the Garden Pond/Large Garden Pond would be nice, though, and should be easy enough for SE to create.