"Tall drink of water" is an idiom meaning that whatever or whoever the phrase is describing is a lot to handle. That it will take a lot of effort to handle, especially at one time (in one gulp).
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[url="http://movies.yahoo.com/shop?d=hc&id=1800019830&cf=pg&photoid=145583&intl=us "]She[/url] is a tall drink of water.

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Sometimes it's also used to refer to a person. I'm not entirely clear about the meaning, but I think it's a humourous way of describing someone as being tall.
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no, above not correct.
"tall drink of water" only refers to someone who is tall & slender.