Wow. I love swimming, scuba, snorkeling; this would just totally change the face of the sport. Perhaps more interestingly, with great care, new techniques for decompression could be implemented so that divers can stay deep for hours on end. With gradual, staged resurfacing, meticulously calculated, all of the excess nitrogen could be depleted from the blood safely.
I'm imagining weekend underwater vacations. Lengthy explorations of South America's cenotes, and the blue holes. Caving and wreck diving would offer entirely new experiences.
On top of that, we get an entirely new form of cardio exercise. Even for people who just don't swim well can start doing their aerobics classes at the bottom of the swimming pool.
For me, karate sparring at 40 feet under... aaaah that would be just a whole new experience.