Mt Arapiles rock-climbing area has a few options for swimming on a warm day, which is perfect after some climbing adventures!
Both options have a rope-swing, and easy access to the water from the car (less than a 10 meter walk).
A little cold, but really nice on a hotter day. On the other side of Natimuk (about 10 mins drive)
Drive down Polkemmet Road, then turn down Obrees Road until just before the river, where a dirt track goes through the trees for a hundred meters or so.
If there’s water in it (usually the case outside of summer), this is the closest option to Arapiles.
Drive down Arapiles-Grass Flat Road, and then Three Chain Road, and go around the dam until you’re at the far end with the rope-swing.