If the image has the continents on a transparent background, just set the alpha-lock for the layer, and then bucket-fill with the new color/pattern (no selection needed).
Otherwise:
- start the fuzzy select tool
- click on the oceans, to create a selection on them, and maybe shift-click on isolated oceans/seas/water bodies (Mediterannean, Black Sea, Caspian sea, Aral sea, Baltic Sea, Great Lakes, Baikal, Balkash...(*)) to add them to that selection
- Select>Grow by one or two pixels (the selection should enter the continents a bit)
- Colors>Color to alpha and remove the water color.
You should then have the continents/islands on their own layer, and you are back to the previous case.
(*) Some of these are geographically connected to the oceans, but on your map the connection could be too small for Gimp to notice.
Otherwise:
- start the fuzzy select tool
- click on the oceans, to create a selection on them, and maybe shift-click on isolated oceans/seas/water bodies (Mediterannean, Black Sea, Caspian sea, Aral sea, Baltic Sea, Great Lakes, Baikal, Balkash...(*)) to add them to that selection
- Select>Grow by one or two pixels (the selection should enter the continents a bit)
- Colors>Color to alpha and remove the water color.
You should then have the continents/islands on their own layer, and you are back to the previous case.
(*) Some of these are geographically connected to the oceans, but on your map the connection could be too small for Gimp to notice.