I am a 65 year old active male, 5'-11' tall, 215 pounds having no known health issues. I swim every day and prefer to swim underwater holding my breath as long as I can while swimming. Is this activity harmfull to my heart?
Thank you in advance.
Jim

Do you hold your breath and not let any out until you surface?

I advise to not hold your breath the entire time, as you can become dizzy, nauseated, or even risk "drowning"

If you want to swim underwater, the best way would be to breathe out while swimming.

Controlled breathing/rhythmic breathing are essential for under water swimming, but when this kind of breathing becomes too forced it can be deadly. So when you hold your breath for longer than 1-2 minutes under water you are reducing oxygen levels in your bloodstream. I let small amounts of air out as I swim.