Flooded quarries are prone to wild claims, in particular about “hidden currents that drag you under” and the usual freezing water.
The water in quarries behaves in a similar way to lakes – it warms in summer and, if the water is deep enough, thermal layers might be present. Flooded quarries are spring or stream-fed, and of course water from underground is relatively cold in summer so you might find a patch of cooler water where the spring or stream enters. This will not kill you, but do be aware of it.
Quarries do not have hidden currents beyond the flow from filler streams and springs, and there are no currents that will drag a swimmer under the water.
Potential hazards in quarries include industrial pollution (it’s worth investigating what was quarried there), and underwater hazards such as rocks, cars, old bikes, and usually a legendary body or two. So don’t jump or dive in unless you’ve thoroughly checked the area first, as you would for any swimming hole.
As always, check there is a suitable and easily-accessible entry and exit point before you get in. Quarries by their nature have steep sides. They are often very deep, and it’s easy for weak or non-swimmers to stumble out of their depth.