(John) swim in the sea throughout the year. Where I swim depends on the wind and the weather. The monthly logs of my swims, links to various past swims including Panama swimming club, South London Swimming club, Boxing Day and new Year's Day swims can all be accessed by clicking on the links on the left. Currently the water here in West Wales is still realtively warm (12 deg C) at the end of November.
 
There are several good beaches in Ceredigion (Cardiganshire for those not living here!). My favourites are:
Aberporth - faces North East believe it or not so is safe most of the time. This is where I swim most of the Winter. Sandy
 
Picture shows Aberporth in winter with afrost on the beach
Mwnt - a National Trust beach which can have a lot of washed up rubbish in Winter and does sometimes have freak peaky surf which is quite dangerous. Sandy
 
 
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Penbryn - Nice when not stoney. Currently there is not much sand there
Pwll Gwaelod in North Pembrokeshire has agood beach although the sand is quite black.
 
Broadhaven (South) and Barafundle are both beautiful beaches to swim from south of Pembroke
Picture shows Barafundle Bay on 23rd November - what a beautiful place to swim