The pilot of a twin-engine airplane related that after landing on a wet paved runway, the airplane veered to the right. The airplane went off the right side of the runway and struck a sign, sustaining structural damage to the right wing. The pilot reported in his written statement to the NTSB that there was standing water on the runway, and he believes that the airplane hydroplaned after landing. He noted that there were no preaccident mechanical anomalies with the airplane.