The outline comes from water_surface.tga which I didn't edit to bother. If you don't like it, you can replace it with a single color or something ;)
Anyways, did you think of changing descr_terrain.txt to reflect the new map size?
There could also be a discrepancy between some of the files. They need all to have the correct size, otherwise all you have to change is map_regions, map_heights and map_groundtypes (the rest can just be resized, staying mono-color for testing purposes)
You could try only adding small bodies of land at a time instead of the whole thing.
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