Thanks for the reference to the pics of the AA587 rudder. The NTSB report stated that entrapped water was detected in the lower portion of the rudder, and the locations of the water appeared to correspond to areas where the facesheet had separated from the honeycomb. It was the opinion of investigators that the water was most likely water from Jamaica Bay.
The NTSB considered five possible scenarios for the cause of the initial failure, before deciding that fracture of the right rear main attachment lug was the most probable one. Two other scenarios (for which there is no detail) were rudder skin fracture, and actuation of a bent rudder hinge line resulting in rudder fracture or rudder hinge line failure.