These techniques have been useful to me as I play with Photoshop, I hope you find them useful too! They work on Photoshop CS2 and CS3, I cannot speak for other versions, unfortunately, though I imagine they'll be much the same.
1.) Taking black lines off of a white background (without the mess):
(Also known as "the secret art of turning a scanned image into transparent lineart" although that is hardly this technique's only application.)
So, you've got a black and white layer, and you want to get rid of the white. (Preferably without turning your lovely black lines/landmass into a jumbled mess of pixels in the process.)
1: Select all and copy the contents of the entire layer.
2: Create a new layer and fill it completely with black.
3: Click the "Quick Mask" button all the way at the bottom of the tool box, or press "Q."
4: Paste. It should now show your black and white layer, with black where the white was and red where the black was. Lovely.
5: Now press the quick mask button (or Q) again. There should be a big selection separating the black from the white. Of course, this is on your all-black layer, so the whole layer will still be black. To fix this...
6: Press delete. This should delete everything on that all-black layer that was white in your original layer, leaving the bits that were varying shades of grey before (between the white and black) as varying shades of transparent now.
2.) The magic wand tool is evil.
So, now that we've got our lovely continent on a separate layer from the sea, what happens if we need to select it, or the sea? Again, our trusty magic wand fails us. Do not be fooled - the magic wand is evil! Simply ctrl+click a layer to select all of the pixels (and their varying transparency) in that layer. Invert the selection (Select -> Invert or Shift+Ctrl+I) to... well, you know.
The technique of removing black lines off of a white background was one I found a long time ago while scouring deviantART for a way to turn my penciled pictures into transparent linearts without having to trace the entire picture onto another layer by hand (with a tablet or otherwise.) I'm still not sure a lot of digital artists know it exists (that, or it's a well-kept secret) - a lot of tutorials simply don't even cover it at all. Most of them either expect you to already know how or expect that you just drew it straight into Photoshop with a tablet. (That works for some people, but I like the feel of a piece of paper - the plastic against glass of my laptop's tablet screen just is not what I'm used to, and I can't seem to draw correctly with it. It has something to do with the way the stylus slides across it, whereas with pencil lead against paper there's some texture. There's a chance I might be a little picky.)
The ability to ctrl+click a layer to select all of the pixels on that layer was something I stumbled upon purely by accident and haven't seen it in many tutorials where it would be extremely useful, so I thought I'd throw that in there.
Hope it's useful! And come to think of it, I hope this is the right place to put it. D: