Now scientists reading DNA have discovered in a period of only a thousand years the "sea nomads" of Indonesia--the Bajau, who live their lives almost entirely on the water—have developed hardwired adaptations to the life aquatic. Several genes have been identified that give the Bajau spleens that 50% larger than average. This allows them to use oxygen more efficiently so they can dive for several minutes on a single breath.
Although not part of the study, they also have developed a very effective style of kicking to swim to down to the seafloor, which you can see in the video. As sea levels rise due to climate change--atmospheric carbon levels are now at a record 410 ppm—the Bajau may find themselves at an advantage. Waterworld anyone?