Originally Posted by Wikipedia

The Japanese independently developed shallow water torpedo modifications ("Thunder Fish") during the planning and training for the attack in 1941. Wooden fins were added to the tail and anti-roll "flippers" kept the torpedo upright once in the water. The fins kept the torpedo's nose level in the air and broke off when they hit the water. The resulting level flight of the torpedoes helped keep them from diving too deeply into the mud (but, despite these modifications, some Japanese torpedoes did indeed dive to the bottom and several remain unaccounted for).