You could do it just the opposite, and make the islands rising up from a flat baseboard, then paint the flat baseboard island shore outline with a sand and sea color paint, then place real "water" overall, then sink the sea floor around the islands.
There are several islands HOG blank routes on the DLS by fishlipsatwork
Someone was going to make a trans-atlantic chunnel route, connecting the US to Europe, I wonder how that turned out ?