Unless there is something wrong (as in pathology) with your lungs you cannot train THEM, increase THEIR efficiency, nor increase their capacity without a significant amount of work that is of little benefit. Lung capacity is essentially genetic, gender, and size related.
You can do resistance training but for any kind of normal activity it provides no particular benefit.
The lungs are incredibly efficient at exchanging gasses. The bottle necks are at transportation and RESPIRATION (the exchange and utilization of gas at the body/cellular level)
VENTILATION is what people think of as breathing. They get mixed up in general use all the time. They are not the same.
When you get 'short of breath' it feels like a lung issue but is really a cardiovascular problem in moving the O2 in and the CO2 out.
Scuba does interfere with VENTILATION due to the increased density of gas moving through.....very.... small airways . It's a physics thing. Helium helps as it's a small molecule BUT it is NOT O2 nor are divers normally O2 deprived, just the opposite most of the time (I'm ignoring rebreather or mixed gas mistakes here as not relevant).
Resistance training and increased capacity will do little to improve the issues associated with diving.
You can train yourself to slow down (decrease metabolism), breath more effectively as depth increases (slowly and deeply), and become somewhat more tolerant to increased CO2 levels (which has a price to pay).
What you CAN improve is the cardiovascular system to be more efficient at transportation and cellular respiration, and tolerate higher levels of the various byproducts from aerobic and anaerobic exercise.
Fritz
Respiratory Therapist, 35+ years
I trust I have some functional knowledge of the field.