In "Abandon Ship" account is a report of the Indianpolis having received a radio message of a merchant ship being damged.
Confirmation:
On July 28, 1945 the I-58 attacked/damaged USS "Wild Hunter"
http://www.combinedfleet.com/I-58.htm

From what I can gather the USS Wild Hunter was an armed cargo ship being escorted by the destroyer Lowry. The Wild Hunter, which my dad was on, was not hit. He said he felt (and others also) that he had hit the Kaiten. The Lowry rammed and sunk the Kaiten that was attempting to attack it, receiving minor damage.