Torpedo attacks on docks, canals, and a dam

How often were torpedoes used against dry-dock gates or canals?

They were used with some frequency, and Swordfish even torpedoed and damaged an Italian dam.

Torpedoes were used against fixed maritime infrastructure more often than their association with ships suggests. A Swordfish squadron even attacked an Italian dam, damaging it without achieving complete destruction. The aircraft's very low speed created an unusual fire-control problem because powered anti-aircraft mounts traversed too quickly to track it smoothly. Italian crews sometimes had to revert to hand-cranked control, turning slowness into an accidental defensive advantage.