Ahhh thanks Wubble :)
Don't worry about sorting out the water around Mammalia, seeing what you've done gave me the idea to have a string of coral reefs along the border, with the one block height difference being resolved as a tiny waterfall over one side of them.

You can select non-blocky boundaries with the wand - I think you want a world edit command something like:

//sel polygon

then it is left click on the tool for the first point, and right click for following ones. The shape must be convex naturally, but I've used it in the past for smooth(-er) biome boundaries.