I have Ecuadorian hermit crabs, they live almost entirely on dry land, but need one or two water dishes to keep their gills moist. I have one dish with freshwater and one with salt water. I also put natural sea sponges in with freshwater, they dig into the sand under them and sit for hours. There are some pics of the crabs and their setup HERE. They eat fruit like plums and apples, plus they love popcorn, cornflakes and peanuts.
They require enough to get above the waterline. I wouldn't say a lot. Jut enough to walk a little. There are also underwater oasises that do the samething without having to lower your water line for above water rocks or pieces of wood or whatever.
