While playing through the original campaign, I've noticed that it's raining inside - specifically in one spot inside the mercenary barracks in Port Last as shown in the screen shot below. It's also occurring all over the place inside almost every cave as if Neverwinter Nights thinks caves are an outdoors tileset when they should be indoors.
Anyway what I'm wondering is this likely an issue with the skybox override I'm using going places that it shouldn't or an issue with the tileset having indoors spots flagged as outdoors thus allowing it to rain where it shouldn't. I would have passed this off as a leaky roof except that it's nice and sunny when I go outside plus the issue with rain in caves.
Although I have a tileset override going, I'm just using the textures from it - NWN should still be using its default .dds, .mdl, .set. and .wok files.
edit- Ignore the apple logo on my desktop as that's just part of that menu. Computer is a PC, Windows 10. I'm not trying to run NWN on a mac.