Dived submarine are detected at range by two means
Sound emissions and heat plumes.

On the surface they are detectable by more conventional means.

So submerged one wants to be quiet and not excessively hot.
Futher one wishes to have long endurance.

Assuming conventional power this means either batteries or burning fuel.

Running diesel under water..oxygen..well either way liquid ox is compact
Noise…yeah diesel are loud
Heat..around two thirds of your fuel goes up as heat.

Now sterling engines
Quiet
Thermal efficiency approaches fifty percent…less heat.
Higher eff. Means less fuel ,less o2 and co2
So sterling win if you want to run engin while submerged.

Of course if batteries were better..send out sub with fully charged batteries and dispense with heat engine entirely.

And then there’s fuel cells…fuel and o2 in…co2and water out…plus a little electric. .wonder why no one though of that? …