Powerbreather and Endless Pool
This combination of using an endless pool and the powerbreather doesn't get much better. You're able to swim as long as you want while breathing fresh air. To swim with a center snorkel where you have pendulum breathing mixing CO2 with every breath would be dangerous at long periods of time.
Work on technique, focus on your stroke, relaxed your neck, breath easy, breath fresh air!
Here's a video demonstration.