> Swim in cold water

This makes your body easier to gain fat. (I did cold water swimming. They warned about this effect.)

I have read countless counter points to that (and some supporting it). Doesn't look that clear cut.

The counterpoints, afaik, amounts to "it consumes tons of calories". Which in fact it does. You then also get really hungry after. And when I ignored that hunger I got sick or was trembling and cold for longer, it was purely dumb to ignore it.

People who do winter swimming are not thinner then average. People who start don't seem to be getting thinner. The advice was to be mindful of what you eat after. But never seen advice to do winter swimming with intention to get thin.