So I remembered where I had read about Kannon (although this describes a different temple than the one I saw):

''[Kannon] is a goddess. She was born in China nearly three thousand years ago. Her father desired her to marry, and on her refusing ordered her to be decapitated. The sword of the executioner shivered across her neck, and did not harm her, so she was condemned to be smothered. When her spirit reached the abode of the unhappy, its presence rendered the place so enjoyable and beautiful that Ema placed her on a lotus-flower, and sent her back to earth and life. Soon after [Kannon's] return her father fell sick, whereupon she fed him with the flesh of her arms. The authorities gave orders for a statue to be erected to her, but the sculptor, instead of merely representing her with two arms, gave her a thousand. For many centuries there was no temple to her in [Toyko]; however, one day a Ronin noble, who, in order to earn his rice, was reduced to fish in the Sumida River, was hauling in his net, when he found in it only a small figure of [Kannon], which he returned to the stream. He repeated his cast in a number of places, and always drew in the same figure. Struck by this singular occurrence, he quitted fishing, and taking the goddess to a shrine that stodd near the present temple, deposited it, and from that grew the mighty fabric we now call the Temple of [Kannon]. [As for the lone fisherman] he renounced his occupation, turned bozu, and died a saint.'' --- Young American in Japan, by Edward Greey