The water is so clear that there are at least 5 shades of blue at the different depths. Then the island is completely surrounded by coral reef, and variety pack of colorful fish.
Underwater
Upside down jellyfish
The reef is a good depth to enjoy with just snorkel gear. Everything around the island is either just below eye level, or free dive-able.
Read more: http://www.gettingstamped.com/2013/07/25/snorkeling-at-silk-cayes-placencia-belize/