Actually, I would probably set up a paludarium, and make the water brackish (about 1 tbsp of marine salt for every three gallons of water) The water area doesn't have to be large- just a couple inches deep. Would really let you get crazy with aquascaping and driftwood.
There are actually two types of "Hermit" crabs in the hobby, and both prefer salt in their water. The hermit crabs that carry a shell on their back do well with brackish to full marine water, while the other hermits (that don't remove their shell- they look like other crabs) like both types of water.. (brackish and fresh, in two different areas)
Both need high humidity as well, so a paludarium would keep you from having to mist them every day.