In reply to The Potato:
Bala Lake Llyn Tegid, river Dee in a number of parts, all the way to Chester. Llyn Geirionydd, Lyn Padarn Fairy Glen or some of the other mountain lakes.
Lots of choice, may need to be discrete and change plans
Look up a few groups on facebook, I'm a member of Chesterfrosties, and whilst most of our swims are Chester and Shropshire, we do also venture west
Post edited at 12:59