There are patterns that show up, the problem is it looks like they are like you said generated by the map seed. If you make the whole map sea, and then place a single land tile, none of the sea tiles around the island change their number. Showing that once the map is generated then the sea tiles are fixed.
This possibly could be used:
1) Make a map all sea, you will sea where your additions have been placed.
2) Place the land strategically to make used of the additions, also placing land or ocean over top of areas of additions you want to go away.
Sounds a little backwards.
But if you don't like the placement of the additions you could always generate a new map with a different seed.