It seems the music is linked not to the latest texture, but to the underwater texture. There are usually two types of water: either water created by the level (which isn't a texture, so the music comes from the ground texture), or water-texture, which is used to create rivers, lakes, and shallows. It's similar to lava and swamp, although movement is generally disabled on lava.
Swamp can also have its own music track; it seems to be in the resources as Terrain-Swamp.ogg, but it isn't used, and the water track plays instead.
I spent about 2-3 hours in the map editor trying to figure out what's what and learn everything you wrote (about the types of water and how they are created/work).
But I'm just a fool and don't know anything; I thought everyone else could do these things in 1 minute with their skills. )The funniest part is deleting water; I spent a long time clicking everything before I figured out how to delete it.
And then I found the second boss – paths! I might be too stupid, but I couldn't figure out how to delete them for half an hour!
Because paths basically work like water, but in reverse. There's also a ground texture under the path, and the path is on top. And I kept deleting the ground under the path, not understanding how to delete the path itself.
I fiddled with it for half an hour, but I figured it out.