Around 1900. They were held in very dubious regard in the early days of development.

https://www.usni.org/magazines/proceedings/1941/january/chap...

On British naval luminary compared submarine warfare to piracy, leading to the emergence a few years later of a tradition of Royal Navy submarine captains flying the Jolly Roger after completing successful missions.

First Sea Lord Admiral Wilson famously called submarines "underhanded, unfair, and damned un-English." Yet this didn't prevent the RN from purchasing submarines from the US in 1901, far earlier than most other industrial nations.