I have never been to a cardiologist, I hope I don't have to go to see one !
About the fins I have been using them for some time and never noticed anything particularly problematic. Maybe it's the way I kick...
Well, the time to see one is before you have an event, not after. According to you, something is "wrong" and unless that was a poor choice of words on your part, better listen to the doc (tsandm).
Anyways, some of this is kind of funny to me. I am in fact very fit and yet if I were to jump on my bike and peddle furiously or jump in the pool and swim at maximum speed, I assure you it would not be terribly long before I too would think something was wrong. Nobody can sustain maximum output, which is anaerobic. You will have to slow down into the aerobic zone. Your buddy can go longer or faster because he has a higher threshold at that shift point that you for possibly several reasons, he could be more fit, his kick or equipment might lend to greater efficiency or there could be something wrong with you.
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