Our marina in Clear Lake TX had very little water exchange. Flushing the head with saltwater would almost drive you out of the boat. Friends with other B40s used their shower wand to provide fresh water for the flush. We had looked at a well equipped Beneteau 46 and Sailboat Mike, our broker, told me several times that the forward head was convertible between saltwater and fresh. After we bought our boat, I found out why this was such a big deal. Mike let me spend some time looking thru the plumbing to see how this was done. I reviewed the Jabsco toilet data sheets for salt and fresh water flush and the only part numbers that were different were in the water feed to the head. Also read Peggy’s book several times but she does not address electric head issues. I purchased the Jabsco fresh water supply 12 vdc electric solenoid valve with integral VENT. I think it was around $90 4 years ago. Cold head sink faucet water line provided fresh water supply, solenoid has vent, so it was mounted up high just under the counter top. The water output of the solenoid valve is teed into the line between the saltwater supply thru hull and the pump. A quarter turn ball valve is after the solenoid in this fresh water supply line before the tee into the saltwater line. As the saltwater pump can create a bit of pressure, you DO NOT want the possibility of saltwater going backwards thru the solenoid. House water pressure has no trouble flowing thru the pump. I elected to manual switch electrical connectors so that either the pump or solenoid could be run off the existing water fill switch. I did not want a switch that control the pump or solenoid valve that might accidentally be in the wrong position. Close the thru hull and open the fresh water supply ball valve, connect electrical to solenoid valve and fresh water flush. Close the FW ball valve, open the thru hull, and connect electrical to the pump and saltwater flush. One other way the fresh water can be used. If the thru hull is opened for a few seconds during a FW bowl fill, the fresh water will back flow thru the thru hull. Closing the thru hull then has the entire line full of fresh water. Update: 2018 - Have switched over from fresh to saltwater in the Bahamas. At some time in the future, I will add a ball valve between the fresh water supply line tee and the solenoid valve. Being able to shut off water pressure to the solenoid would probably be a good idea.
This is probably a three to four hour project if you have every thing on hand. Hope this helps. There is so much stuff under the sink, a clear picture would be difficult to get.