Yet, I've been told on a different sub forum I should be able to roll out of bed and do 400, 600, or 800 with no problem anytime. Swimming is a sport and a skill, unlike the other 3 stamina exercises. It takes months if not years of practise to get those kinds of times.
You were told that if you are to really be worthy of the title dive pro you should be able to roll out of bed and do those swims with no trepidation (or sum such). If one stays in shape and is truly a water person, the months/years of training as a child are all that is necessary.
A DM is expected to be able to save people in the water. If you are only able to finish a 400m swim, what would you do if the boat was swamped and you needed to help someone 400m to shore?