The Watch option has 3 settings.
The default option is ‘Notify me by email when a thread that I have contributed to receives a new message’. The other options are ‘no notification’ and ‘Notify me by email whenever a new message is posted’.
A user can change their individual setting by clicking on the Watch link at the top of the Forums page. It is not possible for a site maintainer to stipulate what the ‘watch-level’ is on their site.