In about 6 years. I've seen a few snakes drinking. Its not very common to see, since they do mostly everything when no-one is watching.

One time my very first female had flipped a water dish while I had gone on a week vacation. When I came back she had no water, So I filled it back up and grabbed her and put her head right in front of it. Her nose hit the water and she started drinking, while I was holding her. LOL

You dont need to soak them to make sure they are drinking. If they are eating, they should be fine on their own. They get alot of water from a meal as well as the nutrients.