yea. that's why locals usually don't talk about their favorite swimming holes.
I visited Dorset for a wedding, and learned of the marble quarry by driving past it on the Manchester road, rte 30.
It was late August, I expected it to be downright freezing, but it was fine--I would guess high 60s, and some sunny spots at the surface in the low 70s at midafternoon. Jump in at the north end, or ease in at the southern end, it's a good swim either way, and pretty to see the quarry from the "inside". A fair number of folks picnic there even though they don't swim, there's a nice grassy area near the roadside, and easy parking too.