M-403 often acted as an escort ship for various ships due to the heavy anti-aircraft fire it could deliver. In early 1945, it had escorted S/S Ostmark when it had loaded mines in Swinemünde and was on its way to Copenhagen.

On April 19, 1945, M-403 served as an escort ship for U-251, U-2502 and U-2335.

In the afternoon, it was attacked by British aircraft and M-403 was sunk shortly after U-251. Those who did not die in the attack froze to death in the cold water.

Today, the wreck of M-403 lies on the starboard side. There are many places to look in and many things to see. When you swim along the wreck, several cannons tower up and they all point to port. In the stern, you can see one of the propellers.