I don't doubt that there's some adaption happening. Freediving has a significant mortality rate that would play into this, but it's also worth mentioning that the world record holders come from all over the world and the numbers are way beyond what anyone would guess if they never heard of it: 13+ min underwater, 24 min with pure oxygen, beyond 200m depth, 300m distance. For an animal that is so clearly adapted for long distance running we are surprisingly good underwater and it might explain why so many people work towards becoming such efficient floating devices during their lifetime. (this is a fat joke)