I second the quick release option.
https://www.amazon.com/Garmin-Forerunner-920XT-Quick-Release/dp/B00O5E7Q0O
It’s $20.
I have a 910, but I suspect that it’s identical.
For pool swim, I use a Garmin swim watch, but in races I use the 910.
I have yet to lose it.
You just have to pay attention when twisting it on and make sure it locks.
If you want, buy a Profile Design Universal Mount:
https://www.amazon.com/Profile-Designs-Design-Computer-Mount/dp/B003UWFZQE
This is my exact set-up.
Swim and run, it’s on my wrist. Bike is on the bars.