The detection of moisture in organic solvents is very important before their use in water-sensitive reactions. Herein, we report that coumarins D1 and D2 were able to generate the corresponding water-sensitive copper complexes D1-Cu and D2-Cu in common organic solvents, which can be used as efficient fluorescent turn-on sensors for water. Single-crystal diffraction analysis of the reaction product indicated that the sensing mechanism is based on the formation of a water-bridged 3D supramolecular hydrogen-bonding network. We demonstrated that a hydroxyl or amine group substituted at the 7-position of the coumarin framework played a key role in the water-sensing performance of D1-Cu and D2-Cu, by acting as a hydrogen bond acceptor in the supramolecular network. This provided a general strategy for designing such coumarin-based copper complexes for fluorescent water sensing. The water-sensing behavior of D1-Cu and D2-Cu were determined to be fast, pH tolerant, and sensitive. As low as 0.0525 wt % of water in methanol can be detected using D1-Cu as the sensor. Moreover, D1-Cu was successfully used for moisture sensing in real commercial products.