calci sand is great, but tends to clump when wet, so as long as the sand stays soft enough for the crabs to dig in, then you'll be fine. Many crabbers only use calcisand as a food source and not substrate, but it's completely up to you.
Baking hardware sand is just to error on the safe side. I had a bacterial infection attack one of my molters which we think may have come from contaminated sand, so I am always extra careful and always bake.
The saltwater I was refering to is not harmful to crabs. I mean the aquarium stuff and it does retard molds.