1) I wore the Garmin FR610 in my swimcap, this is tried and true for getting the most accurate data on a swim.
2) All the other watches were worn on my wrists.
3) The results of which are:
A) Garmin FR910XT: 1.15 Miles
B) Suunto Ambit2: 1.47 Miles
C) Bryton Cardio 60: 1.26 Miles
D) Actual Distance: 1.27 Miles
4) Thus, in this particular circumstance the Bryton unit was the most accurate.