Wow, she must be pretty well adapted to cold water swimming.
Snowdonia is absolutely gorgeous and I've been swimming in a few Llyns myself.
It's certainly not a good idea to go doing that on your own in the winter though. Some of those high mountain lakes are very deep and barely get above a few degrees even in summer. If your not used to expecting the cold, then your body can suddenly go into shock and you can drown very quickly, even if you're a strong swimmer.
Great little film though. I wonder how she did the underwater shots on her own?