I don't think running saltwater through a toilet is straightforward. You would need higher quality parts, more frequent valve maintenance, and downstream waste processing equipped to handle the salt.

Just as a reference, 80% of toilets in Hong Kong are flushed with seawater: https://en.wikipedia.org/wiki/Water_supply_and_sanitation_in...

Sailboats use sea water in their toilets all the time but using it would require two systems, one fresh and another brackish. Very doable but more expensive. I believe they've also use grey water for toilets and plants but again it requires splitting the systems and having storage.