this naturally led to thoughts of how silly it is that when superheroes or other humanoid characters with gills are portrayed in film, art, etc., the gills are always placed on the neck. this makes no sense at all, since gills of that size would be much too small to collect sufficient oxygen for an organism of that size. it would make much more sense to put the gills on the chest or back. it'd look way cooler, too.
and that led to 1) a story idea, and 2) the realization that soon after medical science made it possible for humans to have gills, someone would want to get theirs pierced.
And I LOLed, because I was suddenly hurting in a body part I'd never thought of having.
and then there's the skin thing. you couldn't have the gills covered in the water, so you couldn't wear a traditional wetsuit. you could of course have one with a cutout, but that would rather reduce the efficacy of the insulative properties. so you'd need to come up with some way for people to stay warm, too. and to protect their skin. scales probably wouldn't appeal to most people, since they'd be scratchy and unpleasant to snuggle up with. you could apply an insulating slime layer before going in the water, but it'd get scraped off too easily and wouldn't protect against abrasive surfaces. so i'm still thinking on that one.
I don't think there's a simple solution-- it's not just a matter of exterior exposure, you're pumping quantities of chilly water into your body. There may be practical reasons (not just a matter of evolutionary pathways) why there aren't any warm-blooded creatures with gills.
I tentatively suggest cold-blooded humans with some sort of metabolic tweak to keep the brain warm.
What's the earliest sf saying that bioengineering involved metabolic changes as well as visible changes?
"Lift goes by the square of a given dimension; dead load by the cube of the same dimension, other things being equal. I might be able to make you a Pegasus the size of a cat without distorting the proportions too much." ----Heinlein, "Jerry Was a Man"