Depending on water temperature (so you're not burning energy to maintain body temp), salt vs fresh (you're more buoyant in saltier water), and body fat composition (fat adds to your natural buoyancy), I can definitely see someone who's in decent shape managing 10 hours. I think the record is over 100 hours, so 40 is definitely going to be well above "normal" but not necessarily superhuman.

I mean it's like walking for 10 hours but with less chafing, hell, you can remain with your head above water with very minimal exertion by just flexing enough chest and neck muscles to keep your mouth above water while lying submerged. I wonder if that's where the 40hr upper limit comes from.

You'd be surprised what the human body is capable of.