Setting declination will give you true north. Setting it to zero will give you magnetic north, same as a mechanical compass.
Very true. I also set mine to zero, because that way it matches my mechanical compass.
---------- Post added November 18th, 2015 at 10:42 AM ----------
While that may be technically true, if you are following directions underwater (like a nav course), one should always be on a calibrated compass.
If you're doing it for a nav course, the first step would be to find out if the directions were written based on true north or magnetic.