@ Blackmojo: You want to use the magic wand to select all the non-white parts, then you invert the selection then you save the selection to a channel. This will be your land mask.
@ karro: I guess I knew how to do it, it's just that layer masks are giving me a headache.
The only thing you can apply to a layer mask is a channel, and you can't really convert a layer into a channel, you can only convert a selection into a channel. And you can't copy a layer mask from one layer to another, so if I end up editing a layer mask and I have several layers that use that mask, I can't copy it to the other layers. It appears when you duplicate a layer, the layer mask is copied along with it, but that's the only way I know of to copy a layer mask.
What I ended up doing for the water is copy-pasting my grass layer mask (which is where my rivers and lake is), bluring it and using that as a bump map to apply to a 50% gray layer.
So here's a copy of my map as it is now.
Attachment 14880
I'm not real happy with the border...I'll have to fiddle with it some more.