How about this?

I added a Colorize to the background and then drew the shallow water under the land with the "Sea" drawing tool.

Yes, the original Color Key matches the canals you drew - in that blue colour.

If you switch off the sheet effects you see them light blue, instead of the ocean. They are cutting (at least on my machine they are), but adding a magenta patch as you did in your screen shot above won't work until you add the second Color Key as Joe indicated.

No sheet effects.

Sheet effects.

Don't delete or alter that first Color Key, or these blue shapes will no longer work. Just add the new default one.

If you search for stalagmite in the C:\ProgramData\Profantasy\CC3Plus\Symbols folder, you might come up with a few. That will give you the location, and therefore the style. There aren't that many, but there are a few.

Same for waterfalls.

It really depends on which parts of the full set you haven't got.

That was the drawback. Krita works well and gives good results, but it takes a couple of days to get used to it.

This video covers the basics in Krita, though you have to use your imagination to see how it can be applied to the kind of tiles we make.

I recommend using tiles that are 1-2000 pixels square, rather than the smaller size demonstrated in the video. Use larger tiles when the pattern is something large, like rocks (when the repeat becomes rather more obvious on smaller tiles), and smaller tiles where the pattern is something fine grained, like sand or grass.

https://youtu.be/kTNRd0gBJ-8?t=76

Krita is better quality, but if you are happy with the GIMP version that's fine.