I did Jarvis Island, the water map isn't that hard.
The hard part is smoothly blending the edges of the new tile into the existing ones.
I extracted the highest level standard tile for that area available, scaling it up, then cropping it to the area of the custom tiles.
Then blur the edges of the new tiles and add the scaled and cropped low-res tile as a new layer underneath the hi res tile.
Then, you have a high res tile that smoothly blends into the surrounding low-res tiles.