Fresh water toilet flush
Flushing toilets with sea water generates a layer of calcium inside the discharge tubes. This gets quite thick over time. It blocked the discharge tube in the owner’s cabin (we have three cabin version) after 18 months. It’s a shitty job to solve. Biggest problem is in the non-return valve close to the holding tank. If cleaned in time you have less of an issue. You can dismount the last meter of discharge tube together with the non-return valve and clean it outside of the boat.

Anyway, it appears that flushing with fresh water reduces the calcium problem.
There is a very simple way to modify the toilet in the owner’s cabin to flush it either with fresh water or salt water (when fresh water is scarce). Fit a T-connector between the water tank and the fresh water pump. Fit a 3-way valve between the sea water intake and the toilet pump. Connect a hose between the T-connector and the  3-way valve. Done. See sketch.