I love swimming in lakes.
Once I took a car out to Beaver Run Reservoir to swim there. I spent two or three hours searching for a place to stealth swim. There wasn't one.
It might be easier on a bike - the main impediment with a car was that anyplace near the Reservoir has huge "NO PARKING" signs or is obviously private.
It might be that they do not allow any swimming in it. They are sometimes strict about that for Reservoirs.
Raccoon Park, to the west, has a OK swimming area. You can get to Raccoon by taking the Oscar Swan ride Potato Garden Run to the intersection of Bocktown Cork Rd and Backbone Rd and going west on Backbone (never been on it, but I can guess from the name...).
I think Raccoon is closer than Beaver Run.
Crooked Creek Lake is north of Beaver run on SR 66 ("get your kicks...")> It has good swimming and the park rangers are incredibly cool about people in the swimming area after the official hours.
There are Swan rides to Ford City and the area that could work. It's about 55 miles from the city, though.
I've thought about doing it as an overnight/3-day camping trip.
Maybe the best is to go up the Yough trail a ways and go swimming in the river.
I'm not sure where would be safe. I'm pretty sure in McKeesport it is a bit polluted for swimming - I might be wrong, though.
It might be OK at the Dravo Cemetary camp site. (22 miles from Oakland)
With a river like the Yough, you have to be careful of current. Different places might or might not be safe depending on the season.
Morain State park has a really nice lake, but that is maybe 70 miles each way. I don't know how to get there by bike.
There was some swimming hole near Perryopolis called "blue hole." It's a quarter mile or so from the nearest road. Other than "near Perryopolis" I don't know where it is. Last I was there, it was filled with underage drinkers.
I love swimming in open water. When I rent a car and do a road trip I often swim in lakes. I have not found a place I can ride to and swim.
Please let us know how it works out.
Mick