The intruder is a cruise ship seen in the picture between the campanili of the Madona de l'Orto (right) and San Pietro Martire in Murano (left). The stacks, the most recognizable feature of the ship, can be seen between San Simeon Piccolo (right) and San Pietro Martire. Thanks Sophie for labeling the campanili and sharing the picture with us! Later that day I saw the ship, the Queen Elizabeth, leaving Venice: