B757 Captain reported a maintenance issue at the gate before departure where water was leaking under the aircraft and from the aircraft cabin from an unknown source. The aircraft was not accepted for flight with the maintenance issue.

Narrative

The aircraft completed routine maintenance and was out of service for many weeks. It operated its first revenue flight and during the flight it experienced a water flooding in the forward galley and the flight deck. The flight deck received water all over the first officer and second jumpseat area. The aircraft was scheduled to operate a transcontinental flight next. The aircraft had large amounts of water leaking from the underside while parked at the gate. Maintenance believed that the forward drain in the fwd galley was clogged. They also found a coffee maker that was leaking water (dripping). The solution was to defer the water in the forward part of the aircraft; defer the coffee makers in the fwd galley and then determine if that was sufficient to isolate the water leak. Maintenance was unable to confidently state that there would be no additional water leaking/issues. Even after the water was shut off; water continued to drip from the underside of the aircraft. Maintenance believed that it was residual water in the drains; but could not be certain. After discussing the issue with the dispatcher; Maintenance Control and the onsite maintenance supervisor- it was determined that the plane would not be accepted with those conditions.