How can drinking water help clear mucus from frontal sinuses?

I doubt it: The presence of mucus in your frontal sinus is the result of a blocked naso-frontal duct. This ductile blockage is the result of a nasal infection. I do not think that drinking water will solve the problem although drinking water is good. The duct leading from the frontal sinus to the nose needs to be decongested. A nose spray, decongestant, or steroids might be considered as early treatments.