It seems like cutting off the American’s ability to transfer ships between the Atlantic and the Pacific would have been a significant blow, and surely the Canal wasn’t as well defended as targets like Pearl Harbor.
It also seems like a much more plausible plan than the far fetched attacks the Japanese did consider. Was there a reason that it wasn’t feasible?
They very much did, starting in 1943 intelligence was gathered and crews of the Aichi A6M Seiran floatplanes designed for use from the I-400 class submarines started training for an attack against the locks of the Panama Canal intended to put it out of commission for at least 6 months. Training was originally for a joint torpedo and bomb strike against the locks but since the torpedo attack was judged to be too complicated it was first revised to include only a bombing run and then, at the suggestion of the squadron leader, a kamikazee strike.
A full scale model of the canal locks was constructed at Maizuru naval arsenal for the crews of the Seirans to reherse the attack. The plan was to take off outside Equador and approach Panama from the south.
Since intelligence reports suggested that security around the canal had become increasingly lax as the war went on and an attack against the canal became less likely it might even have succeeded. However, by this ponit it was summer of 1945 and Japan was very much on its last leg. By June 20th all training ceased and the plan was all set. However, before it could be launched the Japanese learned of the concentration of carriers at Ulithi atoll. Since the assumption was that an attack against the Japanese home islands was imminent, this target receiveed priority, however, given the somewhat complicated route decided upon (via Truk) the submarines never reached their destination before the war ended.
If you want more information, I highly recommend Operation Storm: Japan's Top Secret Submarines and Its Plan to Change the Course of World War II by John Geoghegan