I find GIMP does this better than photoshop does (although I'm sure there are PS users who'd disagree). So I'd suggest throwing it into gimp. Then, using the magic wand tool with a threshold of 0, click in the sea area. You may need to ctrl-click/shift-click a few times to add/subtract areas that should or should not be selected. Then grow your selection by a pixel or 2 so that your line goes to the middle of your blue outline (Select > Expand). Then fill black (you can do this by just dragging the black box from your color selector onto the image window). Then invert the selection (Select > Invert) and fill white. Voila!