It is not waterproof and it is sold as “splash proof” I believe.
Another reason why it is not a great idea to use for using it for vigorous activity monitoring is that the sensors can get overwhelmed and reports suggest that the watch can lock up and freeze.
As I have mentioned many times, Android watches are basically the same as your mobile phone.
Not really intended to be used for sports monitoring.
Heart rate, step monitor and walk tracking is no problem.
Any activity that will make you sweat or constant vibration can cause problems though.
You can read about it in the support thread
https://discourse.fullandroidwatch.org/t/optimus-2-official-support-thread-updated-26-06-21