Earlier this year on vacation I caught the ferry over from Cozumel to the mainland in Mexico and dove the cenotes or underground cavern systems which have the clearest water I've ever seen....about 300- foot visibility. You climb a ladder 30 feet underground into a cavern with bats flying around, and then slip into the water and dive through the passage ways which take you about another 30 feet underwater. The scenery was spectacular.
This guy's pictures turned out better than mine so I'm posting his link to his pics, but if you enjoy underwater or photography, you'll like these. It was quite an adventure.