Hurray I got it to work....I was being dumb (not unusual for me) .... the extra explanation about the threshold slider made it all make sense. I took the liberty to amend your tut to just add that contigous should be checked on the selection and unchecked on the fill....hope you don't mind as that threw me for a bit.
What a super tutorial. It's the fastest, simplest coastline method I've seen yet and you don't get the 'lakes at the edge of the coastline' effect.
I've rated this tutorial a straight 5 and hope that others do the same so that it gets the award it so richly deserves.
I've added this tutorial to the quickstart mapping guide.