At a basic level swim records should be evaluated on Garmin Connect post workout not on the fitness device. This basic change will allow you to stop alienating the majority of your user base and make determining the record efforts very simple.
If you are interested in doing this feature up right however, then there would be records for each of the basic 4 stroke types at any distance. This would still be amazingly easy to determine since the swim workouts automatically break out the efforts into sets with stroke type information.
I was considering buying watches and chest straps for my sons swim team to assist the coach in tracking their workouts. But first I would like to see Garmin demonstrate that they do care about supporting their swimming users.