I also keep a 2 liter full without a cap on it, to top off or change snake water. another thing sketh, clean water is perhaps the most important thing you can have for your snakes. every few days wash/disinfect the bowl as well, being sure to remove any bacterial slime that might form on the dish.

I know they'll drink out of a filthy stagnant pond in the wild but they don't live long in the wild either.