Here is the land/sea masks that I used. They're at full resolution for the process, but that should be simple enough for you to fix. I probably should have just done the whole thing at lower resolution; I might have gotten better results.

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Name:	Land.png 
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Size:	14.0 KB 
ID:	46171

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Name:	RIvers.png 
Views:	206 
Size:	24.5 KB 
ID:	46172