In a rare admission, North Korean state media said a naval vessel sank during "combat duties" off the east coast last month.
North Korea state news agency KCNA said submarine chaser No. 233 fell while performing combat duties in mid-October.
Photos released by the agency showed leader Kim Jong Un laying flowers at the foot of a memorial to the dead, encircled by at least 19 graves emblazoned with the faces of the sailors.
The article did not specify what operation the vessel was undertaking.
Information in North Korea is strictly controlled, and accidents are rarely publicly admitted or closely covered by state media, Reuters reports.
North Korean official media did not say how many died in the accident, but said that Mr Kim had taken "measures to find all their bodies", suggesting a high death doll.
South Korea's Choson Ilbo newspaper said the ship sank during a drill, killing scores of sailors, and that two vessels were involved, quoting an unnamed military source.
The two Koreas remain technically at war since the 1950-53 conflict ended in a truce rather than a peace treaty. North Korea conducted its third nuclear test this year in defiance of UN resolutions, drawing condemnation from around the world.
Pyongyang was blamed for sinking a South Korean navy ship in 2010 that killed 46 sailors and in the same year it shelled a South Korean island.