Do I have to add 'as' in the following sentence?
It was (as) clear as water
I ran a search on ngram and didn't find much difference between the two options.
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Technically yes - you are using a simile, and as you are comparing attributes you really need to say it is "as [x] as [y]". If you are finding examples where it has been omitted, these are probably just examples of colloquial speech where care has not been taken.
You could say instead:
It was clear like water.