The command sudo systemctl status watchdog shows it is "dead". The command sudo systemctl enable watchdog didn't helps. The command sudo systemctl start watchdog starts it (and it works), but only until reboot.
$ cat /etc/default/watchdog
# Start watchdog at boot time? 0 or 1
run_watchdog=1
# Start wd_keepalive after stopping watchdog? 0 or 1
run_wd_keepalive=1
# Load module before starting watchdog
watchdog_module="none"
# Specify additional watchdog options here (see manpage).