I'm not sure what it's called but there's a lake as you come around the back of snowdon with a campsite that reaches down to it's edge, there's a cliff face on the other side of the lake which is about a half mile swim across and that's got some good ledges to dry off/jump off but there was a bit of a party on once where I had my first water based fall, lost one manky old mocasym but had a great time.