According to the manufacturer, the Amazfit is IP68 certified.

6 - Dustproof
8 - Protection against permanent submersion

This means that it would be dustproof and waterproof and would not have any problems swimming.
However, I have not tested it myself.

According to an external forum, it is recommended not to stay in salt water or chlorinated water for long periods of time.
They also recommend not diving deeper than 5 metres with it.