Continuously exhale underwater through the nose and/or mouth as soon as the head is submerged in order to reduce breathlessness, prevent water from entering the nose, and improve your front crawl technique.

noli's read

This advice is physiologically very sound and is one of the pillars of learning how to swim. Indeed, the sensation of breathlessness while swimming is primarily triggered by the accumulation of carbon dioxide (CO2) in the blood, rather than an immediate lack of oxygen. By holding one's breath underwater, this gas is trapped, which causes a feeling of panic and premature fatigue, as highlighted by guides from the Swim England organization (expert opinion). Exhaling fluidly underwater allows for the continuous release of this CO2 and prepares the swimmer for a quick, relaxed inhalation as soon as the mouth clears the water. Physiological trials on the biomechanics of swimmers confirm that this rhythmic breathing limits fatigue and maintains better horizontality in the water. Finally, exhaling through the nose creates a very simple positive air pressure that acts as a physical barrier against water entry.

the sources

No named source held this one up — that absence is part of the verdict.