I have one... The battery life is currently OK but not great - something like 55 hours. But I think this can be improved with some firmware changes - and the firmware is open source.
are those batteries available anywhere, or just form them?
They are garden variety 3.7V 200mah 402030 pouch LiPos, which are pretty easy to find. More hardware details here: https://watchy.sqfmi.com/docs/hardware