The pilot and the operator's director of maintenance (DOM), were on a postmaintenance check flight. According to the pilot's written statement, while approaching to land, the throttle control became inoperative, resulting in a partial loss of engine power. The pilot selected a tidal beach as a forced landing site, but was unable to reach it, and landed in shallow water short of the site. The airplane sustained substantial damage to the wings and fuselage. In a written statement to the NTSB, the DOM reported that while troubleshooting a recurring complaint of low engine manifold pressure, he noticed that the castellated nut, which connects the throttle linkage to the fuel control servo, was loose. He said he removed the cotter pin, tightened the nut, and reinstalled the cotter pin. A postaccident engine examination revealed that a self-locking nut was missing from the throttle linkage arm, resulting in the disconnection of the throttle cable. The DOM said that the missing locknut was not the same one that he removed, tightened and reinstalled, but was a nut on the other end of the throttle linkage arm, about 2 inches from the castellated nut that he tightened. The DOM noted that he should have checked the security of the hexagon self-locking nut, but he did not. The hexagon self-locking nut was not recovered.