What replaces the submarine as a torpedo threat

How would naval warfare change if the submarine had never been invented?

Torpedoes would lose their most effective concealed platform, forcing navies to develop another way to deliver a primary ship-killing weapon.

Surface-launched torpedoes became less dominant once torpedo-boat destroyers matured. The submarine restored much of the weapon's strategic menace by concealing the launch platform and threatening targets far from a surface battle line. Removing submarines would not make torpedoes useless, but it would deprive them of that renewed reach and surprise. Navies would have to seek another platform or another primary ship-killing weapon to fill the gap.