Actually the Shinano was to be a sister ship to the Yamato! But converted to a Massive Aircraft carrier. I think it was sank with ONE torpedo! There was NO crew to stop the flooding and the ship went down.
There was plenty of crew, 1435 of them were killed with only 1080 rescued.
Problem was a B-29 recon of Tokyo was seen by everyone in the city, they panicked and sailed Shinano to be completed in the Inland Sea without all the watertight doors fitted and unfinished bulkheads so water spread throughout the ship.
Shinano was hit with four torpedoes. You may be confusing it with Taiho which was hit by one. It likely would have survived had some fool of a damage control officer not turned on the ship’s ventilation system, thereby spreading gasoline vapors through the ship and turning it into a floating bomb waiting for a match.
The ship was intercepted by Archerfish because (a) one of the shaft bearings was running hot, slowing the carrier down about one tenth of a knot, so that Archerfish was able to intercept her some hours later, and (b) the Japanese admiral recalled the destroyer escort that tried to counterattack Archerfish after the sub broken the a cardinal rule of submarine warfare by using its radar. By my lights, the sinking of Shinano was the result of a series of miracles favorable to the U.S. Navy.