Regarding depths, as a casual free-diver I can assert that at roughly 10-12 meters underwater it is a lot easier to stay down for a long time. I suppose this is thanks to compression (with about 1 atm pressure) of the chest so your buoyancy lowers and you don't expend strength and oxygen to keep pressing yourself down in order to "just float", as happens at smaller depths. So if fins or anchored rope helps me get that far down quickly, I can spend maybe twice as much time actively snorkeling than at a couple meters under surface. It does take a greater effort to swim back up though :) but jumping from seabed, or a few flaps with fins, take care of that.
One wet-suit I had had positive buoyancy, so diving in it was a very exerting effort :) I did not have weight belts, they could make sense. 537
In colder water however a wet-suit can make a great difference for staying in water longer (spending less sugar/fat to warm up instead of to move), and note that deeper layers are often much colder than the top foot or two.
In many waters in fact having an extra layer of skin can make sense also as protection (think poisonous jellyfish of New Zealand/Australia region where even a silk-thin suit can save your life); swimming boots (or fins) may be important for foot protection against corals and sharp rocks.
This answer and further discussion at Quora: https://qr.ae/pGkl6J also details how swimming suits for sports helped reduce drag and make energy spending more efficient for swimming - to the extent that superb suits were forbidden for the sake of competitive sport. I suppose it too can help your humans stay underwater longer, so can be a part of the answer even without biology modifications.
FWIW, my "specs" are not stellar, though impress people nearby, with 2+ minutes sitting underwater holding on to something, or about half a minute swimming 25-30 meters across a pool (without fins).
Regarding further specialization, AFAIK some fluorine based solutions act as artificial hemoglobin equivalent. There were some solutions for medical injection/transfusions, known as "blue blood", as well as ones you can "breathe" into your lungs for fancy underwater oxygen-CO2 exchange, like illustrated in The Abyss movie. As an upside, that gets you rid of an air bubble that deeper sea pressure can crush in, so you are same-density water in and out and are much sturdier (no idea if that is pre-saturated with oxygen for divers; I doubt water per se has enough oxygen solved for a warm-blooded mammal to breathe). 671