As a retired Navy Search and Rescue Swimmer, I think I can help!
During training, we used to swim 50 meters -wall to wall under water. Easy, right?! Now do it again... and again... and AGAIN!
The key is going as deep as you can. That will help greatly.
You are actually exersising your lungs. Do it as often as you can, and you will see an improvement. Good luck!
