Stroke count, pool lap times, if devices has indoor swimming support. Of course this is moot for casual swimmers. Regarding outdoor swimming support, the devic

Stroke count, pool lap times, if devices has indoor swimming support. Of course this is moot for casual swimmers. Regarding outdoor swimming support, the device has to support openwater swimming, because then it contains algorithms to smooth out the GPS signal (it goes off when arm goes inside water). Other option is putting the device below the swimming cap, or a swimming bouy that you drag with you. Heart rate is a different beast. Your Polar worked below water because it used their own proprietary signal that can be recorded under water. Garmin uses ANT+ which doesn't work there. They do have a special HR belt called HR Swim (and HR TRI for outside swimming), that pairs to device, then records HR on its own while swimming. After workout it uploads the recording to the watch which combines both data together. Only top triathlon devices support those.