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Pretty much what it says on the tin.
Presume nuclear power doesn't develop, for whatever reason. (Radioisotope generators may be allowed.) So, too, no nuclear weapons, so missile boats aren't an issue.
Presume also you start with subs as effective as possible as early as possible.
So, fuel cell subs in the early 1840s? Stirling-powered boats in the 1830s? Closed-cycle stirling in the 1850s or 1860s? Peroxide turbine boats by 1900?
The aim is best submerged endurance. Low hazard is also good.