In my log on the Garmin web site I see my swim sessions, with distance, time and average pace.
For example, I did 1500 yards in one hour. It shows an average pace of 2:02 min/100 yards. That's wrong. It should have shown 4:00 min/100 yards. It's simple arithmetic: 60/15 = 4.
I actually know that the distance and time are correct. The pool is 25 yards long, the laps count is about the same as my own count, and everything is set properly in the watch.
And 4:00 min/100 yards would also be much closer to my actual pace (and is till on the fast side ... )