Or they actually liked to bathe and a good romp in the water felt great or they learned basic hygiene or they found they could cool off on hot days by jumping in the water.
Or maybe they were in the water fishing.
They invented the "cannonball". Neandertal descendants wrote the theme song for "Petticoat Junction", btw.
No, these growths only form when you habitually submerge in cold, windy environments. So they weren’t taking a dip to cool off, and probably not getting in the water for leisure.
You make a good point that likely goes missed by most modern humans. I have a homestead/self sufficiency farm and don't really have any equipment so everything is done slowly by hand. There were days when I was so hot after doing hard work all day that I just couldn't cool off it seemed no matter what I tried then I couldn't sleep all night. I started taking the family down to the beach in the evening to swim in Lake Michigan and that always just sucked the excess heat right out leaving me relaxed and able to sleep that night. I bought a kiddie pool and to this day when I don't have time for the beach I fill it with well water (about 48 degrees) and soak in there for a bit.
Imagine, you've spent the day engaged in intense physical labor hauling logs for the camp or butchering and carrying mammoth primal cuts back to base. You are a being with a blocky build that doesn't shed heat well and now youre covered in dirt and blood and have to get a good nights rest because you have to do it all over again come morning (along with Mrs Og not appreciating sleeping with all the additional flies you bring home). Theres only one method of treatment and that's a trip down to the swimming hole.