Yup, Diamond is correct. In GIMP the feature is "Canvas Size" (Image > Canvas Size). Now it's been a long time since I've looked at RobA's tutorial, and I can't recall if you are using the same clouds layer for both the land and the ocean or if these are two different layers. If the latter, then Diamond's suggestion should work without modification. If the latter, you will need to somehow generate a selection of the land (e.g. if Rob's tutorial has you create a Land or Sea mask you could use this to make the selection), Inverse the selection, and delete the clouds covering the sea (and the sea color), alter the canvas size of said layers, fill with new clouds and color and voila.

Cheers,
-Arsheesh