They need to be able to come out of the water to get air and they do better in brackish water. There is a difference between surviving and thriving. Do what's best for the crab or don't get one.
Ok i you don't need to be rude. I just asked people that has kept them to see if they need air. I just said i know someone that has kept them without air, that doesn't mean i got my answer. Don't comment back if your just going to be rude.
At the very least I can say they desire air, I had one back in 1990( I didn't know any better) and was in a 20 gallon. They whole time he wanted out until he succeed one day. Found him 2 months later dead under some furniture.