Besides fitness tracking, what's the benefit you find for wearing a watch inn the pool? I used to wear my polar in the pool with my strap, but when I got a Garmin a few years ago it couldn't reliably track my heart rate anyway, and the GPS went wacky from being on the end of my arm when I was out on ocean swims.
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.
I'm a competitive (masters) swimmer and I've always thought it would be nice to track average lap times etc., but then realised that without this being correlated to the set being done would actually be a total gimmick as you couldn't derive anything useful from it.
I just wanted basic fitness tracking (heart rate, active/rest periods, estimated calories burned) that I could use both on traditional workouts like running/walking/biking and swimming. So one platform instead of land-based workouts being tracked and swimming going into a blackhole.
I don't do it competitively, just basic maintenance and general wellbeing.
Finally, it being a watch is a bonus since I will not have to remember to put it on before starting.
You want to know the time when you're swimming?