On July 31, 2017, about 1230 Pacific daylight time, a Cessna 172S, N957TA, was substantially damaged in an accident about 5 nautical miles northwest of Seattle International Airport (SEA), Seattle, Washington. The flight instructor and student pilot receiving instruction sustained minor injuries. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 instructional flight. The flight instructor reported that after completing air work they were returning to Boeing Field/King County International Airport (BFI), Seattle, Washington, when the engine started running rough. During the descent, the engine started to run rougher and it continued to lose rpm. Shortly thereafter the low oil pressure light illuminated, and 10 to 15 seconds later the engine quit. When the pilot attempted to restart the engine and the propeller would not move, he elected to ditch the airplane in the water close to the shoreline; the landing resulted in the deformation of the engine mount. The following day the airplane was recovered to a secured facility for further examination. According to a Federal Aviation Administration (FAA) aviation safety inspector who was on site during the recovery process the day following the accident, as the airplane was removed from the water and the engine cowling was removed, he observed that the top bolt of the right magneto was loose and had backed off, which created a 3/8" gap between the magneto and its mounting pad. An external visual examination of the engine revealed no catastrophic damage; however, the engine was seized and would not rotate. The muffler was damaged slightly, the right magneto was canted aft from its mounting pad at the top right position, with the attachment nut observed loose. The lower mounting nut was loose at the bottom of the mount. There were 4 accessory case bolts missing from the accessory case, three from the right side at the bottom of the case, one on the left bottom and one bolt on the left side was protruding halfway out. When the loose right magneto was removed from its mounting pad, there was no noticeable damage to the magneto gear, although there were metal flakes observed inside the magneto, and its gasket was torn. When the left magneto was removed, it was observed to be tight in its mount; metal flakes were detected on the gears. The remainder of the accessories were removed from the accessory section of the engine. While no damage was observed to any of the accessories, metal flakes were detected within and around them. After the accessory case bolts had been removed, a visual inspection of each bolt revealed no stress signatures to their respective threads. An examination of the right magneto's top right bolt that had backed out, revealed that no stress signatures were observed on the bolt's threads. During removal of the accessory case bolts, it was observed that more than half of the bolts were not tight. The manufacturer's maintenance manual stipulates that all bolts should be properly torqued; the bolts have a star lock washer, however, no safety wire. No stress was found on any of the bolt threads or their respective mount threads. Each bolt had the appropriate washer. When the engine case was disassembled, it was observed that the crankshaft had broken in half at the middle bearing. A large section of the crankshaft had separated, which was subsequently located in the bottom of the case. When the crankshaft was removed, it was observed that the bearing had become welded to the shaft. Additionally, heat stress signatures were observed. A representative of the engine manufacturer reported that based on the findings, it was his opinion that when the right magneto came loose and backed out of its mount, it allowed all of the oil to exit at the magneto mounting hole. Maintenance records revealed that the zero-time rebuilt engine was installed on the airplane, December 29, 2015. At the time of the accident, the engine had accumulated a total of 917.5 hours, with 22.4 hours since its last annual inspection, dated July 25, 2017. It was noted that both the left and right magnetos were removed and overhauled on November 3, 2016, during a 500-hour inspection, which was 497 hours after the engine was installed, and 420.5 hours before the failure of the engine. Maintenance records also revealed that 122 hours before the accident, the accessory case was removed in order to replace the oil pump body, as well as the drive and impeller. When the case was removed during the examination of the engine, there were no anomalies noted with any of the previously referred to components.