As a kid I have memories of driving a bit out the edge of town to a big steel plant for almost exactly this reason, it's not totally insane.

There's plenty of free parking, free admission, giant pool with some small waterslides and stuff, a bunch of picnic tables and public barbecues, generally some nice greenery and trees for shade, ponds and fountains and stuff, a bandstand, etc.

It's not somewhere you'd pop over to for a "quick swim", but especially for lower income people it's a great place to have around as a "grab a pack of hotdogs and your swim trunks and make a day of it" sort of thing. As a kid we couldn't afford to, say, go to the actual waterslide park or anything so I have a lot of fond memories of visiting the steel plant.

I'm sure the construction and upkeep was less than a rounding error in terms of construction and upkeep costs for the plant itself.