Lots of behaviors impair your driving ability. A study found that being dehydrated caused the same number of driving errors as being drunk [0]. I'm sure it would be easy to make a field sobriety test for blood water content, but they don't, because safety isn't the real reason these laws get made. It's just public pressure to enforce moral values.
[0]: https://www.telegraph.co.uk/news/health/news/11547199/Not-dr...