It only takes a few generations to induce noticeable change, if the pressure to adapt is strong enough.

Grow something in a garden. Arbitrarily choose some trait which varies across your first crop and selectively breed for that trait. The first generation will be statistically affected, but perhaps not obvious to you if the trait you've chosen is a recessive one. Even with recessive genes, after 3 generations of this kind of aggressive selection process, the plants will have obviously changed in favor of your selection. It may be a subtle change, but it will be widespread, and statistically obvious.

The problem with the situation you describe is that no aggressive selection process is in place. It'd only work if that tiny number of guys who can hold their breath for a long time were the only guys allowed to breed in that society, and only with the women who could hold their breath the longest. This would have to go on for a few generations, so something like 50 - 100 years.

The problem is that humans are mostly offended by this kind of eugenics being imposed on them by other humans. Also, humans are generally willing to alter the environment rather than not breed. So the guy who invents SCUBA gear may not be able to hold his breath for very long, but can effectively work as though he could for even longer than Mr. Blueface.