I would think most snakes drank soft water except those from desert areas.
There are lots of areas with hard water that aren't in the desert. Most areas with a lot of limestone will have hard water and they aren't necessarily dry areas. I don't know for sure, but I will be the water in the FL Keys is fairly hard.
I don't think it presents a significant problem to snakes any more than it does to people. People may not like the taste of water in a particular area (ours is very soft but still tastes disgusting!), but I have never heard of any physiological complications from drinking hard water.
Now getting hard water deposits out of water bowls can be a pain, but that is why you should use disposable deli cups for water bowls!
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Chris Harrison
...he was beginning to realize he was the creature of a god that appreciated the discomfort of his worshippers - W. Somerset Maugham