I need water.

Meaning: This means you're saying that you require water to drink because you're thirsty or because your body needs it to function properly. It's like saying, I'm thirsty and I need to drink some water.

Choice of Words: Need means to require something that is essential or necessary. Water is a liquid that we drink to stay hydrated. When you put them together, "I need water" means that you require water to drink in order to stay healthy.