English Translation
Water is wet.
Ser or estar?
On the internet there seem to be lots of examples of both es and está for this (or similarly mojada instead of húmeda).
I’ve learnt that the choice, generally speaking, is a matter of perspective: ser is used to express an objective fact, whereas estar is used to express a personal experience.
In this case, I wonder if the English translation shouldn’t rather be “The water is wet”, referring to a specific body of water. 
Of course, I’m not a native speaker of either Spanish or English, so you had better add some salt to the water. 
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English Translation
Water is wet.
Interestingly, Google Ngrams only recognizes “el agua es húmeda”.
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