Sounds like dehydration issue. What are your temps? Indigos can thrive at room temp for much of the year in temperate climates. They do not need temps above 80 F. What substrate do you have in your moist box to hold moisture? Moist sphagnum works great. Lots of it. For snakes still in racks, I just fill half the tub with moist sphagnum.
If it is truly dehydrated, just soaking in water will not resolve the underlying problem. I question if your caging is just too hot and dry?