I’m in a wetsuit but there are a hardy few in ‘skins’.
Clothing choice is incredibly personal, depending on your body fat (a few extra pounds keep you warm), how well you acclimatise, and so on.
Cold water swimming isn’t a sport you can dive into
If you’re not acclimatised, ‘cold water shock’ can be fatal, and severe hypothermia can be lethal too. Consequently, everyone has swam from the summer through the autumn. By the time it gets to winter, you’ll already know what you’re comfortable wearing.
And there’s a wide range of options: ‘skins’ (if you’re already highly experienced in cold water), a summer wetsuit or a winter wetsuit. Plus a choice of regular vs. thermal swim cap, neoprene boots and gloves.
Oh, and don’t forget your dive watch