The middle of the chest and the lower rear lobes is where the most of the volume is. The most expansion comes from the back of the trunk from mid back to the waist here is where the lower lobes allow for the most expansion, however because of the restrictiveness due to tension in the lower back can restrict the expansion. This is a problem and we deal with it in this way.

Sit down and be erect but not stiff, if you'd rather stand bend your knees slightly so as to unlock them.
Relax your jaw and let your gut go.
Put your thumbs below your back just above your pelvis. This is the kidneys. Wrap your hands around your love handles with a firm grip. Then breathing through your nose breathe in slow and lengthy for a three count. Allow it to expand your fingers forcing them apart. Feel the tension.
Now breathe out and count to seven. Be careful not to tighten your belly while you extend the breathing out. Just slow-down the speed of exhalation.
Do it again 3 count in 7 count out.

Now if you are getting dizzy or spaced out then rest a bit.
Take the lead from the breathing lung. Try and match its pace.

 

Once this feels comfortable over the coming days or weeks extend the exhale count. Attempt to get to twenty. Remember do not tighten up the gut.
eventually you should feel a calming influence when you do this, and your lung capacity should expand. You'll feel better and have more energy to burn allowing you to survive under the water being tossed around like a cork but remaining calmer and more focused.
A good length of time for this exercise is around the 10 minute mark.