If you let your coconut sit in the freezer overnight, it will be much easier to open up the shell. Simply hit the frozen coconut with the dull side of a cleaver a few times, and it will crack in half. Then, after you scrape out the frozen coconut water, the shell can be easily popped off the coconut meat.
There's a little trick that makes it even better: just pop cut chunks of watermelon into the freezer for about 30 minutes, long enough for the surface to firm up while the inside stays juicy. If you want it to ice up evenly, spread the chunks on a flat surface, like a cookie sheet or a plate. The result is a naturally icy, slush-like bite, essentially a popsicle without the stick.