Thanks for the quick feedback!
I assumed nylon would be a better alternative because it's resistant to most liquid chemicals (from what I hear).
And ABS doesn't exactly "appear" watertight or water resistant -- VS. how smooth and durable nylon looks after a print. Perhaps it's because I've been printing ABS at the 300 micron level. It just seems like water will seep through the lines and break down over time.

I'll keep in mind the number of shells. Nice to know it doesn't have to be 100% infill.

I've read from several sources that PLA will break down in water btw. Probably not an ocassional splash, but supposed if it's subjected to being underwater for long periods.

I primarily use ABS however, so if I can do most anything with that, I'll stick with it.

Does anyone know what kind of plastic is used by commercial manufacturers for toilet parts? I assume it's plastic injection molding, but what kind of plastic?