i also have a watermelon crab. i got it two days ago at wal-mart. i am have the same problem as you. i have no idea what to do with it! i have searched everywhere for info. but there is none to be found! i know a few things so here goes. it is fully mature when it reaches about 1 and a half inches. it does not stay fully submersed but is consistently climbing in and out of water. it eats just about anything, loves fresh veggies, and right now i am experimenting with hermit crab treats. i am feeding it baby shrimp currently. but i cant really tell if he likes them yet. i know they like to burrow tunnels in and out of the dirt. but my problem is bedding. the pet shop man told me to use hermit crab sand instead of potting soil. but im thinking that was dumb. because he didnt know what a watermelon crab was, and because since i changed it my crab refuses to come down off his rock. i am trying to find out if sand is ok for it, but no luck yet. let me know if you find anything. hope i helped a little...