Why is Quarry water usually cold?

I would figure that since they are surrounded by rock, they are isolated from most of the natural heat by that barrier, keeping them at a cold temperature. They are also deep and that would contribute as well.

A lot of quarries are deeper than the local water table. When the quarry is in operation, pumps are used to keep the mine dry. But, when the mining operation stops/ends ..there’s no longer a need for keeping the quarry dry. An inflow of cold ground water then floods the quarry. Because this inflow is continuous, the quarry water stays very cold ..even during the hottest weeks of summer.

A quarry is a type of open-pit mine from which rock or minerals are extracted. Quarries are generally used for extracting building materials, such as dimension stone, construction aggregate, riprap, sand, and gravel and I guess this is only reason why water in quarries is cold.

All of the above is correct. Additionally, quarries are huge compared to a swimming pool and there really isn’t a place where you can stand up. If you get tired, when you swim to a side, there may not be a place where you can get out or hang onto the rock. I’ve been swimming a quarry before. The shallow part was 40 ft. deep.