Maldives
Credit: Michele Westmorland / Getty Images

More than 3,000 reefs grow among the 20 atolls of the Maldives and, along with monsoon tides of the Indian Ocean, create one of the world's richest underwater habitats. Few places have whiter sand or bluer water, and the sea remains warm and clear all year, the better to observe more than a thousand species of colorful fish – snapper, pufferfish, wrasse, trevally, clownfish, banner fish – as well as micro-marine fauna, manta and eagle rays, and sharks. Some of the best sites are near Kuredu Resort, which offers a variety of snorkel trips. This island nation rises only six feet above sea level at its highest point, and the whole thing could be underwater by 2100, thanks to rising sea levels due to climate change. Better go now.