I was thinking of this, too. I swim in the water here down to 6 or 7°C in a 7mm wetsuit. It's cold, but it's very bearable and I enjoy it comfortably for the fi

I was thinking of this, too. I swim in the water here down to 6 or 7°C in a 7mm wetsuit. It's cold, but it's very bearable and I enjoy it comfortably for the first 45 minutes or so. If I'm diving a lot (I'm usually spearfishing down to 20m or so) I'll often extend that quite a while due to generated heat. I'm not sure what it is, but the exertion of diving combined with breath-holding turns my body into a furnace. When I first started doing this, I found the water in summer (around 6°C warmer) incredibly cold. I'd shiver if I was snorkelling for a while because my activity was too low. I struggled with that because being so cold would ruin breath-holds. Now I kind of prefer winter dives, because in summer I get unbearably hot. I constantly need to let water into my suit, and I can swim without it for around 1.5 hours. That adjustment only took a year or so. People are awesome at adapting to environments in that way.