I think it would take a few weeks at least before people realized what was going on. A few potential drownings would be survived, and the news would go crazy with all these people who can breathe underwater. Then scientists would study it like crazy, and then eventually test it out clinically. (Man, imagine the headlines when they announce that shit? "Guess what everyone! We can all breathe underwater now!")

Anyways, beaches would become even more popular, and real estate prices near the coast would skyrocket even more than before. There would still be some obstacles towards everyone just becoming amphibious, though. Plenty of places have water that's very low quality. Pollution in coastal areas, rivers, and lakes would become a huge issue. It's bad enough to swim in dirty water, but imagine breathing it! Also, shark attacks and other aquatic dangers would become more prominent, just because the higher volume of people in the water, now. Underwater personal protection devices would be a necessity if you planned an extended exploration of dangerous waters.

Other people have mentioned the crushing pressure at the bottom of the ocean, but I don't think it would be that big a problem. ~90% of all the interesting shit in the oceans can be found in shallow waters. That's where all the life is. Any place where people could snorkel freely without diving equipment would suddenly become much more accessible. Anyway, there would probably be huge ramifications to the global economy. The scuba industry would lose money, true, but lots of other industries could stand to make a LOT of money. "Water Hotels" and "Water Condos" would become a thing in areas that aren't hit by hurricanes. Water sports would probably become more popular. Many things would change, but most of those changes would be awesome.