I went for a swim at the marvellous London Fields Lido on a cold, grey day in London. It was absolutely pouring with rain, and areas of the park were flooded.
That didn't stop people coming for a swim. There were plenty of people at the lido. Several people cycled through the rain to swim here.
The water is heated to 26 degrees Celsius, so it's pleasant while you're swimming, but a coolish walk to the hot, outdoor showers.
I swam 10 lengths of front crawl and backstroke, then floated along for another couple, just looking at the winter skies above the pool. There was so much steam coming off the water, that, at times, it was like swimming in fog.