You can log your swimming sessions using the Workout app on your Apple Watch. Select a "Pool swim" workout and follow the instructions. You can find further instructions on how to use your Apple Watch for swimming on Apple's website : https://support.apple.com/en-us/HT209393
Sometimes, the application isn't up to date with the last workout if it was in the background. If so, try restarting Swimr. A pull to refresh feature is on the works !
Swimr currently only works in meters. Using the device locale units is the very next feature to be implemented.
Apple Watch is a trademark of Apple Inc., registered in the U.S. and other countries.