Not a good idea to use sheetrock in pond with fish or where you are going to swim due to fiberglass fibers.
http://www.usg.com/Design_Solutions/2_2_6_diffdrywall.asp From this web site above:
"Type ‘X’ gypsum drywall, such as SHEETROCK® Brand Gypsum Panels, FIRECODE Core, has glass fibers mixed with the gypsum to reinforce the core of the panels. These fibers have the effect of bridging and reducing the extent and size of the cracks that form as the water is driven off, thereby extending the length of time that the gypsum can resist heat without failure."