It's covered with water plus occasional mountains, mud flats, rocks etc. except at the south pole, where there's a small amount of land that isn't continually being swept out to sea by the tide. Basically, I've colorized the globe of Ganymede and added the land mass at the south pole without making any major changes.
A Ganymedan Nympus offers to sell his catch, an amphibious land-leet (left) and an aquatic sea-leet (right). Both are edible by humans, but the land-leet is generally considered "good eating," the sea-leet is not.Holiday Magazine 2090
I've actually followed the description of the nympus as closely as I can - it's come out looking a little strange, but the mushroom head, webbed arms, etc. are part of the story. Remember that this will be a fairly small image in the final text, so some imperfections and edits won't be so visible.