I have always believed the Utah-mistaken-for-a-carrier explanation was a canard. The photo of Utah heeling shows nothing which would promote the idea she could have been mistaken for a USN aircraft carrier. First and foremost, no USN carrier had a cage mast. Even if this was lost in the fluster of a combat mission, Utah still had tall barbettes and protective structures built over the 5-inch turrets on the barbettes. Her silhouette did not remotely lend itself to a mistaken identity. A USN carrier did not resemble a short, lumpy, old, unreconstructed USN dreadnought. Her decks were not visible from the low-angle attack attitude of the torpedo bombers.
I think the best explanation is that the torpedo bombers, which took the highest mission-type casualties in the raid---the belief the USN was caught defenseless is simply not true---there were .50 caliber gunners (the only ones ready to fire) on the ships, particularly, up high in the masts of the battleships, who were shooting at the Nakajimas, and inflicting casualties, were not inclined to do a go around, but took the best target in their line of sight. To a man under fire, or seeing a near aircraft plunge into the water, "that object" looked like a battleship, damn the finer points, and they launched their torpedoes. After all, one torpedo bomber pilot mistook Helena for Pennsylvania; this, in itself, is as big a stretch as mistaking Utah for a first-line battleship.