Could humans ever evolve into marine animals?

If there were a selective pressure of some kind that drove a human population to an increasingly aquatic lifestyle, we would eventually evolve accordingly.

And this has already really happened in the Sama-Bajau ethnic group.

The Bajau spend most of their day in the water and have acquired some quite astounding characteristics that help them lead this lifestyle.

They can hold their breath for as long as 13 minutes thanks to a spleen twice the size of an ordinary human. During diving, the spleen shrinks to release additional hemoglobin, saturating the blood with oxygen. An adaptation that dugongs and seals have also developed. They also see better underwater than the common man, and their constitution is slender, thus requiring even less oxygen.

Obviously, to have something similar to the humans of Atlantis in the Aquaman movie, the required mutations would have to be even more radical.

To obviate low temperatures, they could, for example, exploit a vascular structure that tuna and penguins’ feet also possess, called the rete mirabile, an intricate tangle of blood vessels that preserves body heat within the body.