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.