You might think so.
But, of two merchants and a destroyer and a carrier and an amphib, all 20 miles off but close to each other near the shore, how do you aim a torpedo at the right target?
Because the acoustic signature of a large, single screw merchantman is different than that of a large, twin screwed warship which is, in turn, different from a small twin screwed warship. Any sonar man worth his salt can identify targets by the noise they're putting out alone and target the torpedo accordingly.