Jesus speaks about giving someone a cup of water as an example of how one person may help another. This is a metaphor for helping someone in any way. (See: [[rc://en/ta/man/jit/figs-metaphor]])
This negative sentence emphasizes the positive meaning. In some languages, it is more natural to use a positive statement. Alternate translation: "definitely receive" (See: [[rc://en/ta/man/jit/figs-litotes]])