Played with master layers a little more. It seems that you have to lock the master layer by clicking on the pencil icon to gray it out before switching pages. Otherwise, the contents of the layer would jump to an active regular layer. Strange behavior. Also, it is the grid layer that determines whether the master layer's contents is over or under the regular layers for the various pages. If the master layer is under the grid layer, your watermark is under everything. It is over everything if it is above the grid layer. And you can add additional master layers so you have one above and below the regular layers. Very handy if you're planning on using the same background for multiple pages...