I mainly cobble maps together in GIMP. I only played around with the early versions of CC ages ago, and Dundjinni a little after.
Can you import textures and such into CC? If so, do you know about the CSUAC? I have no idea what the acronym stands for anymore, but it’s a huge honkin’ collection of textures, objects and such gathered for mapping.
Also, the Cartographer’s Guild? Really helpful forum of mappers on pretty much every platform. There are a lot of helpful folks and tutorials there as well as insanely cool maps.
The Underwhere does indeed stretch under the oceans. It can pretty much go anywhere you want it to, although we need to consider the availability of air and such under the ocean. Maybe have the area under the oceans be mostly flooded and inhabited by subterranean aquatic creatures.
Air isn’t a problem due to the Flatchulent Fungi that are found through most of the Underwhere that can be heard releasing a mixture of gases including Oxygen (and periodically Helium making for fun times and squeaky voices), though you can without notice wander into a Flatulent Fungi Free zone leading to purple faces and scrambling for exits!