Its only possible to swim in the Thames west of Oxford (where I live) between mid June and mid September. At all other times its too bloody cold.
Its wonderful at the riverbank with a mate, some cigs, a flask of something and just sit.... and watch all the wildlife around you: cormorants, coots, grebes, swans, voles, and some MONUMENTAL fish which kept cruising round and jumping.
We stayed last night until the sun had set. MAGIC.