iserith
Magic Wordsmith
If a PC is trying to push past his or her normal limits and the outcome is uncertain and there's a meaningful consequence for failure, the DM may call for a Constitution check.There should be some significance to distance.
I can swim across a 20 foot pool and back. I could not even begin to contemplate actually swimming even a mile (or half, etc).
Gaining any distance in rough water or areas of thick seaweed or while another creature interfering with your swimming may call for a Strength (Athletics) check, again, if the outcome is uncertain and there's a meaningful consequence for failure.