Water on the inside of any type of glass product exposed to the exterior, such as a window or skylight, is most likely due to interior condensation than exterior leakage. High humidity often occurs in a bathroom, especially when a shower is used. When outside temperatures chill the surface of the skylight or window, airborne moisture can condense into droplets. This is most likely to occur on single pane glass rather then the traditional double paned glass. If a seal around a window or skylight is not sealing correctly and allowing cold outside air to infiltrate to a warmer surface, either in the form of glass or metal, this will also cause water droplets to form. 

A practical solution would be increased ventilation and/or removal of excess humidity from the air. Bathrooms are required to have openable windows or exhaust fans, but sometimes both are needed to eliminate excessive humidity. This type of problem can also occur in other parts of the house annd a de-humidifier may be needed. Once you remove the moist air, condensation (water droplets) should cease to be a problem.