How air-dropped torpedoes could work in shallow water

Why did aerial torpedoes not simply dive into the bottom of a shallow harbor?

A tension-wire system altered the torpedo's entry so it did not plunge as deeply before rising to its operating depth.

An air-dropped torpedo normally dives after entry before rising to its running depth. That behavior made shallow anchorages appear safer from torpedo bombers than they really were. A tension-wire arrangement could make the weapon enter more like a belly flop, reducing the depth of its initial plunge. British experience at Taranto had already shown that Pearl Harbor's depth did not make it immune to aerial torpedo attack.