Although I can’t imagine an Internal Combusion Engine sub being at all stealthy

Diesel electrics can be very stealthy, with the potential to be even more so than nuclear subs when trying to hide (given equivalent level of technology elsewhere in the design). What they can’t do is continue being stealthy for anywhere near the time a nuclear sub can as eventually you need to come near the surface and run the diesel to recharge the batteries. Diesel electrics are also comparatively range limited - while they can travel a considerable distance nuclear subs are effectively only limited by their ability to supply the crew.

As a side note you need the electric part of diesel electric to have an effective submarine. Running a submarine on an internal combustion engine only is impractical as without an adequate oxygen supply you can’t operate underwater without the engine using all the oxygen, so the diesel engines are used on the surface (or just below drawing air through a snorkel) to charge a battery bank which then runs an electric motor when submerged.