If you use a recent version of C::B you can select the variable you want to watch and drag it into the watches window.
Very quick and easy. This instruction should be added to the debug wiki(nightlies section).
Right now the wiki has:
Click in the empty last row in the watches window, type the name of the variable (or full expression) and hit enter.
Which meant that I was doing a lot of typing of variable names in the watches window(up to 15 variables or more), and this was error-prone for long variable names. I couldn't find a way of auto-completing variable names in the watches window - I don't think that this is actually an option?
I'm adding it to my TODO. Thank you for the request.
Is this in your TODO list? :D
The automatic inclusion of local variables and function arguments[in the Watches window] have not being reimplemented. Sorry about the inconvenience.