There is a balance to humidity. A good humidity level is around 50%....but that is hard to have in the winter and in dry places. My humidity rarely goes above 30 or 40%. If your humidity is not over 60%, I would not worry about too much humidity. Do you have a humidity gauge? She may be drinking more often than you think...she may come out to drink when you are not looking. My young garter, Hermes, does not drink in front of me. Some garter snakes do not swim much. Dehydration is only a possibility. She may not be dehydrated. Are there any veterinarians that work with reptiles where you are?

I think it is better to feed in small chunks instead of bigger pieces. It is easier for the snake to swallow and digest small pieces. Fresh fish fillet like salmon, trout, or tilapia is good to feed. Frozen and thawed pinky mice are also good to feed. Buy fresh fish fillet at a seafood shop if you can. Do not cook it. Feed it to the snake uncooked. Frozen pinky mice can be purchased at pet stores. You must thaw them before feeding them to your snake.