Most saltwater crabs are fully aquatic, so do not need to get out of the water for survival. Many BW crabs do need to get ot the surface to breath, but these are not fully aquatic crabs. Emerald crabs, sally light foot, hermits, etc do not need to leave the water. With hermits, it can be bad for them--they can get air trapped inside their shells. This keeps them from being stable, and they tend to die. If you do pull hermits out of the water, be sure to rotate them several times under watedr to force the bubbles out.
There are several trace minerals needed for crabs to survive, but most are provided with a quality salt mix. What all is in the tank with the crabs? There are many fish which will make a meal of a crab immediately following it's molt, so plenty of hiding places is a requirement. Also, inverts in general are more sensitive to instabilities like salinity fluctuations and ammonia/nitrites/nitrates. Careful maintenance will improve your odds of keeping inverts.