Operation Tiger April 28, 1944

Reported here:

When the convoy was maneuvering in Lyme Bay in the early hours of April 28, they were attacked by nine German E-boats out of Cherbourg that had evaded the Allied patrols. No warning of the presence of enemy boats had been received until LST-507 was torpedoed at 0204. The ship burst into flames, and survivors abandoned ship. Several minutes later LST-531 was torpedoed and sank in six minutes. LST-289, which opened fire at E-boats, was also torpedoed but was able to reach port. The other LSTs plus two British destroyers fired at the E-boats, which used smoke and high speed to escape. This brief action resulted in 198 Navy dead and missing and 441 Army dead and missing according to the naval action reports. Later Army reports gave 551 as the total number of dead and missing soldiers.

As noted here:

It was the costliest training exercise in all of World War II. As the bodies washed ashore in days ahead, the official count rose to 749.

But, as also noted,

The brave men who died that day contributed to the success in France six weeks later. Indeed their sacrifice was a Prelude to Victory.