It wouldn’t call it a smart watch but I am experimenting with a Xiaomi Mi Band 2.
The Band isn’t open source but it is possible to send commands to it from Linux.
Things I just tested from my Linux PC:
-) Vibrate/Show incoming Call
-) Vibrate/Show incoming Message
-) Read Battery state (state in percent, last charged date/time)
-) Read DateTime of the Band
-) Read Step Counter
In other code examples I read about custom vibration, color change and set time of the band.
I hope I can make it work for my Librem5.
The band has only basic functions:
-) Time
-) 20 days batterylife (manufacturer info)
-) stepcounter
-) Heart Rate Sensor
-) vibrate on Incoming Call
-) vibrate on Incoming Message