I've had hermit crabs. Husband had some fiddler crabs, but he just dropped them in our fish tank and they eventually died. Hermits did well with a sloped sand-and-gravel "land" that ended up in shallow water (we didn't bother making it brackish, but since it was rural well water it was already chemical-free). From what I've heard, it's important to remember they need something to keep their gills moist without drowning them.
Sorry that's probably not much help. They're cute little devils though!
~Marrow