THE PILOT HAD COMPUTED LANDING PERFORMANCE INFORMATION FOR A LANDING ON A WET RUNWAY WITH ANTI-SKID; SUFFICIENT RUNWAY SURFACE WAS AVAILABLE. TOUCHDOWN WAS NORMAL, HOWEVER DURING THE BRAKING PROCESS, THE AIRPLANE STARTED TO FISHTAIL AND THE PILOT BELIEVED THAT THE ANTI-SKID SYSTEM HAD FAILED. THEREFORE, THE PILOT TURNED THE ANTI-SKID SYSTEM OFF AND CONTINUED THE ROLLOUT WITH NORMAL BRAKING. THE AIRPLANE WENT OFF THE DEPARTURE END OF RUNWAY 05 AND SUSTAINED SUBSTANTIAL DAMAGE. PERFORMANCE DATA INDICATED MORE RUNWAY SURFACE WAS REQUIRED FOR A SAFE LANDING ON A WET RUNWAY WITHOUT ANTI-SKID ASSISTANCE. THE BRAKING SEQUENCE OF EVENTS DESCRIBED BY THE PILOT RESEMBLED DYNAMIC HYDROPLANING WHERE THERE WAS A COMPLETE LOSS OF BRAKING EFFECTIVENESS RATHER THAN AN ANTI-SKID MALFUNCTION. AN EXAMINATION OF THE AIRPLANE FAILED TO DISCLOSE ANY MECHANICAL PROBLEMS.