Could Humans become adapted to living underwater?

I think the easy answer would be: "Yes".

However, it mostly would be a matter of "getting adapted" to being about to do this in addition to any genetic manipulation that would be necessary for doing this.

The other problem that I see today (at least here on earth) is that human pollutants concentrate in the oceans for example, from chemical fertilizers and other chemicals that are unnatural in the forms they make it into nature in the ocean.

So, at least for now, choosing where someone would want to "live" part time or full time underwater might be important just like people would often choose beautiful places with clear air and water to live above ground away from big cities and smog and water and air pollution. Likewise, you might want to choose a cleaner place to get underwater as well.

Also, because of Fukushima the radiation levels of all oceans (since they are all interconnected by water currents worldwide) radiation from Japan's meltdowns is constantly going to be going into the ocean there for the next 25,000 to 50,000 years.

On one level people will adapt over time being in the ocean to this ever increasing radiation. But, there likely will be many fatalities over the next 25,000 to 50,000 years in the process of this genetic adaptation to ever increasing radiation as human beings in all oceans.

The radiation is going to take a whole cross section of humanity out of the genetic pool just like coronavirus is taking many types of genetics off the face of the earth temporarily or permanently.

So, the radiation is a part of human Natural Selection worldwide just like coronavirus is a part of human natural selection worldwide as well.