I like the ships, but I'm not sure that I like them being blended into the sea as if they belonged there. The ships are (roughly) side-on, while the islands/sea map is clearly seen from above, so they're separate elements that are combined on the stamp. I don't think that the ships belong on the maps, as maps. If you're going to detail the water they are sailing through, I would keep the contour lines out of that area to distinguish it from the sea of the map. A little break between the map sea and the ships would also fix the issue of the contour lines and sail creases coming together in the 4th stamp.

To me, it is the top-right stamp that seems a bit cramped, probably because the tops of the masts are nearly scraping against the islands.

I'm also not sure that the ships are quite as crisp as the map elements, which won't be a problem if they aren't blended so closely.

You should probably experiment with price and other labeling before deciding any design changes like aeshnidae's suggestion to move the island or making any adjustment based on my concerns over the upper right stamp.