"Is this water?"
Translation:He wai kēia?
I think Iʻd translate that as "is this the water?" rather than "is this water?"
Ka & ke are definite articles, parallel to "the" in English. He is indefinite, and English lacks that - when a noun is indefinite in English, we just omit the article. So "he" corresponds to the English sentence that doesnʻt have an article before the noun.
Perhaps it has something to do with the questions, He aha kēia? (What's this?) and He aha kēlā? (What's that?)
In answering these questions, we take out the question word AHA and insert the answer in its place. For example: He waiū kēia. He poke kēlā. Looks like a pattern to memorize. Aloha