The more time you spend in the water the more comfortable you will feel. The more comfortable feel the less air you will breath. There is no real shortcut that I have ever seen that actually works.
Actually there is something of a shortcut at least in terms of bottom time. The year before getting scuba certified I logged just over 100 local skin dives. A wet face, sustained oral breathing and being away from shore on the water were second nature. The much maligned snorkel will train you to breathe deeply and deliberately to avoid being winded. You get accustomed to a mask, submerged face, dealing with a wet suit, donning fins, weight belt and nearly all the gear of scuba. Add a BC, regulator and cylinder and you're scuba diving. It's a great gateway sport and equally effective for active divers. Many divers have local sites where they can get this exposure. One of my first winters I made it a routine to go to the Y and do laps in mask, fins and snorkel followed by breath-hold pool lengths. That sort of intense fin kicking really jacks up your cramp resistance.
The other magic bullet is to sing, in the car, shower whatever you want to make some noise. Controlling your lungs for long passage and notes will tell your diaphragm who is boss and give you the tool to hone when in the water.
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Pete