Not got much time to go into detail this minute, but how I usually do it is with just the sea selected, stroke the layer (use a new layer).
Then go to select -> modify -> contract (and pick say, 5 pixels or something. Then stroke the new selection on a new layer. Then contract it again, etc etc, repeating until you have as many as you want.
Then you delete the bits you didn't want stroked (it'll stroke the whole selection after all). Then you add gaussian blur to the layers.
I'm betting someone has a more efficient technique though. =P