I am pretty iffy on making channels do fancy things other than height maps for lighting effects. What I would do to accomplish what I think you're after is:
1. Create a new layer to hold the lighter colour. Ideally this would be above the layer of dark ocean but below the layer that holds the landmass.
2. Fill that layer with the colour you want.
3. Get a selection of the land-mass pixels (ctrl-click or cmd-click on the layer thumbnail for the landmass).
4. Expand that selection by about twice the number of pixels you want the lighter coloured area to be. So if you want 20 pixels of lighter blue around your continent, expand the selection by about 40 pixels.
5. Make sure the new fill layer is active, and click the Add Layer Mask button.
6. Blur the layer mask until it looks the way you want it. You can also play with the opacity and blending mode of the layer.

Edit: Simpler
Okay, after having written that I played around a bit. I think you can simplify what I said.
The first two steps are the same. Then just create an empty layer mask on the fill layer. Now go into channels. You should see a new channel for the new layer mask. Copy the channel you've made for blending and paste it into the new layer mask channel. I think that should work.

Let us know if there's something else that needs doing. I'm looking forward to seeing your map!

cheers,
Meshon