The entire time Lapon operated in the Sea of Japan, she was surrounded by thick fog and had problems with her radar.

After she exited the Sea of Japan through La Perouse Strait, she patrolled off the east coast of Hokkaidō and Honshū.

Lapon cleared the area at flank speed chased by an escort and returned to Fremantle 6 June for refit.

Underway for the eastern part of the South China Sea 29 June, Lapon spotted a cruiser and destroyer, the latter escaping at high speed.

Lapon closed in for a night submerged radar attack and sunk the tanker Tinshin Maru and damaged two other vessels.

Patrolling off the coast of Luzon 10 October 1944, Lapon intercepted a Manila-bound convoy and sank the cargo ship Ejiri Maru.

Lapon left port for her seventh patrol 23 November and took position on a scouting line to prevent enemy reinforcements from reaching Mindoro and Leyte.

Lapon was the last submarine to prowl Lingayen Gulf before the invasion of Luzon, and was ordered out of the area at high speed the day of the first landings.

Returning to Guam 20 June, she headed for Saipan 4 days later to join submarines for a picket boat sweep ahead of Admiral Halsey’s forces which were to attack Tokyo area targets.

On 8 August 1957 she was transferred to Greece by Captain Samuel Francis[9] under the Military Assistance Program and served the Greek Navy as Poseidon (S-78) (originally carrying pennant number Y-16).

Poseidon was purchased outright by Greece in April 1976 and stricken from active duty for use as spare parts for sister ships.