This is easily one of the best swim places we've ever found and we can't believe it's not already on here! The water is a delicious green and the setting is secluded and beautiful. It's even large enough to satisfy those serious OSS swimmers. We'd recommend it as being a place worth travelling to. If you have an OS map it's at about 564, 922. Otherwise, from the B3260 that runs SW from Okehampton to the A30 take the road signposted to Meldon Quarry. Just before reaching the quarry there is a car park on the right with a map and information board. The old quarry pool is a short walk down hill over the stream that runs down from the reservoir. It's straightforward. It seems that on the map there is also a couple of pools adjacent to the car park that might be worth checking out too.
There is a picture of the pool from our visit here: http://www.flickr.com/photos/sally_jane/2752263033
There is also about it here: