In reply to ClimberEd:
Yep both Bala and Padarn are good ones, they're cold though!
if you want the sea then Llandudno or Newborough Beach South Anglesey - both you can swim for quite a distance parallel and not too far out.
Obvious safety comments- don't do it alone and wear a bright hat!!
Fatality in Bala at the w/e with someone falling from canoe