Once you get your landmass how you want it and then merge things so that the land is white and the sea is black on one layer then use Filter - Noise - Median with a setting of 1 or 2 and that will take out a bunch of tiny pixels. OR you could use the magic wand tool to click on the landmasses, then Filter - Select - Modify - Expand = 1, new layer fill with white and merge down. This also takes out stray pixels. OR you could use Filter - Pixellate - Crystallize = 12 (then go back with a 9 a 6 and a 3) then Image - Adjustments - Brightness/Contrast and max the contrast. OR you could make up a new technique. The bottom line is that there are many ways.