I would write it without the hyphen because "crystal" is modifying "clear." Then again, if "crystal clear" is a compound adjective, then you could put the hyphen in. I'm still new at hyphens, but that's my take. Merriam-Webster does show it without the hyphen, though, even as an adjective. Their site also gives "crystal clear water" as an example.
By my understanding, you would only put a hyphen between them if they serve as a single description of one thing. "Five-mile run" would require a hyphen, because five-mile is a single description for the run. Same deal with "Two-year-old child," and yes, your example as well. "Crystal-clear waters" is correct. However, this is an odd description, because it's also fair to interpret it another way. You can assume that crystal is a separate description for clear, describing exactly how clear the thing is. Either way is correct in this case. I'm sure that's clear as mud. Or, alternately, I could say that this has been a clear-as-mud post. Hyphens can be tricky. Whenever you see them, pay attention to how they're used and try to work out why they're used. If you can get the feel for their use, it's much better than memorizing a bunch of hard-to-remember rules.
I'm not sure what you mean about "it modifies 'clear'". In "this is a long-term relationship" the word "long" modifies the term, but it is hyphenated. Maybe I don't understand something... Thanks.
Long-term is a single description. You can't remove the "long" or the "term" and still have it carry the same meaning, therefore, it requires a hyphen. I should have said that in my first post. >_<
I'm not sure if my example was good (although you certainly can say "term relationship"), but here are two other examples that are hyphenated: A long-anticipated decision was finally made. He got a much-needed haircut yesterday. (both "long" and "much" can be removed) http://www.grammarbook.com/punctuation/hyphens.asp I don't see how they differ from "crystal-clear water." Thanks a lot!
I've never heard of a "term relationship," unless you're talking about the relation between two or more words--a synonym for term. The difference in those two examples are that long does not describe anticipated, nor does much describe needed. They're two words coming together to modify the noun, so they may as well be a single word. Whereas with the water, both "crystal" and "clear" can stand alone without breaking a grammatical rule or changing the meaning of the sentence. Or they can sing in one voice. That's why it can go either way. It may not specifically say that in any rulebook, but it's something to look for.
Like so many of these issues, there is no right and wrong. Different style guides will give you different answers, and English doesn't have a legislative body to rule on the disagreements. The style guide I cited earlier would have you hyphenate "crystal-clear waters", but some style guides say only to hyphenate to remove ambiguity, in which case you'd go for "crystal clear waters". Choose a style guide and work to it.
