[Doolar]

The air in the boat will replenish itself, back to 21% after a few minutes of surfacing.

The "breathable air (O2)" will not. If the Chief Engineer need to add breathable Oyxgen to the boat during a long dives and when the Oxygen drops below 17%, that is not replenishable. I've had to use it from time to time and have never ran short. I use the values set by OneAlex. You can adjust these values via the H.Sie Option selector sliders. located in the SH3 Game Folder / Support folder. I personally like the way OneAlex set these values.

Now if you mean the "Compressed Air" for blowing ballist, that will also replenish itself, but it will take some time.

Hope this helps