Bac Lava: A Very large lake with clear clean water. It is deep water but very peaceful. Also nice at night to look up at the stars while floating on your back.
Access is easy - car park nearby.
Stuart Keiller: Waited 4 year to come back and swim here. Well worth the wait :)
Viv Rickman: Easy access from Ogwen Cottage car park via the path. Popular access point from the beach on the Northwest side. It's shallow for a long way out giving you plenty time to get used to the temperature ;-)
More info http://viviennerickmanpoole.tumblr.com/post/62644332248/colds-clarity-crispness-and-a-bit-of
Very weedy at the narrowest part
Nathan Wood: Very cold deep lake. Easily accessible from Ogwen Cottage, as its a 10-15 minute walk away.
Easiest way in is the large pebble beach on the northern shore, hurts your feet a bit mind.
Very cold, dress accordingly.
Nigel Robertson: Ended up here after plan A failed. What a lovely spot, very easy access from Idwal Cottage. Entered from the gently slanting beach over the increasingly bigger pebbles and then you are suddenly plunged into the deepness as your breath is momentarily taken from you. Idyllic, soaked up the atmosphere with the clouds just above my head
It can be quite slippery underfoot - take care!