I have raised an entire clutch of corn snakes in enclosures filled with moist sphagnum, but no water bowls. They all (20 or so) grew to near adult size in less than one year. However, I fed them heavily and sprayed the sphagnum frequently. It is possible they licked some water droplets from the moss.
I believe they got sufficient water from their food and kept well hydrated from the sphagnum.