If you need to make a long surface swim, do it on your back so your fins will stay submerged for their full strokes. Manage currents wisely. Learn how to detect, avoid and cope efficiently with adverse currents. Read an article on diving in currents for more details. However efficient, every regulator presents some resistance to breathing.
Denser air causes more friction.
At two atmospheres, for example, there are twice as many molecules in a given volume of air, and twice as much friction. With every breath, your lungs have to drag this dense air through the regulator's demand valve, through the mouthpiece, and down your trachea to thousands of lung alveoli, then force it all the way back out again past another valve. Remember, friction increases exponentially with speed. Move the air slowly. The more complete each breath is, the fewer of them you have to take.