On June 14, 2021, about 1230 eastern daylight time, a Diamond DA20-C1, N458MA, was substantially damaged when it was involved in an accident near Matthews, Virginia. The flight instructor and student pilot sustained minor injuries. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 instructional flight. The flight instructor reported that during preflight inspection, he added 1 quart of oil to the engine to bring the total quantity to 5 quarts. During a short cross-country flight to Tangier Island in the Chesapeake Bay, air traffic control instructed the flight to descend from 5,500 ft mean sea level (msl) to 2,500 ft msl, due to an active military operations area. After the student pilot leveled the airplane at 2,500 ft msl, the flight instructor noticed engine vibration and roughness that he initially corrected by advancing the throttle from 1,900 to 2,200 rpm; however, the engine subsequently shook “violently” followed by a total loss of engine power. The flight instructor took control of the airplane and realized that there was not enough altitude to glide to a shore. The flight instructor then ditched the airplane in the Chesapeake Bay near a fishing trawler where he and the student pilot were rescued. The wreckage was recovered about 2 weeks after the accident and examined at a recovery facility. Examination of the wreckage revealed that the right wing sustained substantial damage. Examination of the engine revealed that that oil sump drain plug was missing, and the Nos. 1 and 2 connecting rods were fractured in a manner consistent with oil starvation. Review of maintenance records revealed that the airplane’s most recent 100-hour inspection was completed 4 days and 9.3 operating hours before the accident. During an interview, the mechanic who performed the 100-hour inspection stated that he remembered draining the old oil, reinstalling the drain plug with a new gasket, torquing it per the maintenance manual, and securing it with safety wire. He did not remember anything abnormal or any interruptions during the work.