I noticed this while playing a few minutes ago. It was 95 degrees F and sunny, so I started baking as soon as I got out of my car, even though I was in my warm-weather gear. So I got my bathing suit and hopped in the lake next to my mobile home… only to find that I kept getting hotter. Diving under the surface didn’t even help. Since when is the inside of a car on a summer day a place to keep cool, while deep water makes you hot? Now, I realize that greenhouse effects and wind considerations are probably code nightmares that don’t even merit thinking about at this stage, but I think the water side of it could probably be handled pretty neatly.
Suggestion 1: treat the weather as if it’s 10 degrees F (5 degrees C) colder than the displayed Weather Temperature while you’re in Deep Water, in addition to the effects of being Wet on the insulation value of your clothing.
Suggestion 2: If you’re fully submerged (having dived under the surface), subtract another 20 degrees F (11 degrees C), for a total -30 degrees F.
Suggestion 3: Wearing a Wetsuit should negate these effects.
In short: I think that going for a swim should help you cool off in the summer, and it should be suicidal in the winter.