yes, you're right. Use singular verbs for uncountable nouns. In America, also use singular verbs for *collective* nouns. In British English, I've seen both singular and plural verbs used for collective nouns. US: my family *is* smart. UK: my family *is* smart OR my family *are* smart

yes you are right, there is some water

Yes, the second phrase is correct, "is" referred to the water witch is singular.