The best snake water loss information I can find at the moment is this, from 1968. Sounds like there's a big difference between xeric and mesic species:
1. The skin, rather than the respiratory tract, was the major avenue
for evaporative water loss in the two species of snakes examined.
2. The total evaporation of the brown water snake (Natrix taxispilota) was
3'3 times greater than that of the desert-dwelling Sonora gopher snake (Pituophis
catenifer affinis).
3. The pulmonary component of evaporation was nearly the same in the two
species, hence the cutaneous component accounted for the difference in total
evaporation.
4. These results are in accord with findings in other reptiles, which likewise
show a correlation of reduced evaporative water loss to the aridity of their
environment.
Schmidt-Nielsen K and Prange, H (1968.) Evaporative water loss in snakes. Comp. Biochem. Physiol., 1969, VoL 28, pp. 973 to 975.