I would be afriad of the chemicals you could find in it and if it will turn into a goo with so much moisture. Why not just spend $4 for a 50 pound bag of playsand at a hardware store? Or if you can't find any, there are blocks of coconut fibers in the pet stores that you soak in water and it makes the perfect substrate for hermit crabs.
i use eco-earth for the substrate, it holds moisture really well and keeps the humidity high. i've also used playsand from home depot in the past, the 50lb bag. it worked fine, my crabs had no problems. as for the temperature... i dont use a heater at all. in the winter, my house can get pretty cold, probably low 60's, maybe even upper 50's. but my crabs survive. i mist my terrarium like crazy to keep the humidty very high. the move less when its cold, but they'll survive.
Hermit crabs can drown if the water is deep enough.