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I don't swim, but I have done thousands of runs with my series of garmin watches and I can say that the UX for them is spectacular, everything is in a sensible place for me to do without thinking. Not sure what problems you've had with it specifically
Garmin is certainly better than some previous smartwatches I've had. You get a sunlight-readable, always-on display and a week-long battery life. But with my Forerunner, they've packed a lot of options into the five buttons. Leads to a lot of "these buttons are for up and down, except at the start run screen where the up button opens the menu, which you can then navigate with up and down to choose between the six typ…
A Pebble successor has to be better than a Pebble 2. The only reason my Pebble 2 isn't used anymore (and why I swapped to Fossil which is discontinued, too) is hardware buttons died. I tried to donor from a Pebble Classic, but sadly failed.
On top of all this, I get skin rashes from watches, so I cannot wear them 24/7.