If you're like me, when you think of poisonous snakes, you picture a large viper or cobra in a jungle somewhere. Conversely, when I hear "sea snake", I imagine a small, harmless, wiggly creature about 6 inches long. Think again. The Great Barrier Reef is home to 17 species of sea snake, and while most live on the ocean floor, their lack of gills means that they need to come to the surface for air... which means there is a pretty good chance you could run into one while snorkeling. And try not to let it bite you: sea snakes can have some of the most potent venom of all snakes. (The good news is most will bite only when provoked.) Oh yeah, and those in the Great Barrier Reef can reach lengths of 4-5 feet.Happy snorkeling!