Tall drink of water?

It means a large glass (a tall one).

But this is said of a woman. Somebody used to call her a tall drink of water. She was a tall woman, so maybe this has something to do with it.
I wonder whether it is an idiom?

Not in reference to people, no, I suppose they were being 'creative'. Someone can be a 'breath of fresh air' idiomatically, but not a glass of water. What is the whole section of text?

Wild guess: 'hot'?

A large glass of water at meal times is said to satisfy one's craving for food.

MrP

Alternatively she could be cold - to pour cold water on something, icy..

She could be depressingly non-exotic, in comparison to champagne or such

Hello,

Tall drink is a mixed drink served in a tall glass, and I guess as she is a tall drink of water not mixed with anythig else, it might mean that she is a tall blunt lady.

Hello!

This is the context:
Sara was what Jeffrey's father liked to call a tall drink of water. Jeffrey had been attracted to Sara because of her strong will, her fierce independence. He liked her aloofness and the way she talked down to his football buddies. He liked the way her mind worked and the fact that he could talk about every aspect of his job and know she would understand

Then I'd say it's because she's straightforward, plain, nothing fancy, but refreshing.

No I'd say she was cold - aloof, independent, talks down to people.