There's a lot of confusion regarding fresh water heads and raw water heads. Let me try to explain a little:
1. There's no such thing as a salt water head and there's no such thing as a fresh water head.
2. Any head that flushes with water from outside the boat, be it salt water, brackish water, or fresh water, coming from a lake, river, stream, bay, ocean, mud puddle or whatever - is a RAW water flushing head.
3. Fresh water flushing heads are those that tie into your PRESSURE WATER SYSTEM on the boat - the same water that you shower with, wash your hands with, cook with, or drink. They utilize an electric solenoid valve and a special atmoshperic vacuum breaker to assure that the toilet water can never back up into your fresh water system and contaminate it. The solenoid valve opens when you push the button to flush the toilet, like an electric faucet, and pressure water comes into the toilet bowl for rinsing. The valve closes when you let go of the button to stop flushing the toilet.
Raw water flushing heads cannot be connected to the fresh water supply, either to the pressure system or to the non-pressurized reservoir tank without taking a risk of contaminating the fresh water system and you coming down with the heebie-jeebies, cholera, or whatever. The are not designed for that application and should never be used in any application other than what they were designed for.
People, after the fact, will sometimes decide that they don't like their raw water-flushing head because of the odor that comes from the outside water, particularly when in a marina, or the fact that the raw water stains the inside of the toilet bowl, etc., etc., and wish to convert the existing head to flush with pressurized fresh water. It CAN be done with most electric toilets, but it is neither cheap nor easy to do it. It CANNOT be done with manual toilets. Manual toilets are designed for raw water only, and can never be safely converted to flush with fresh water from your pressure system or from your fresh water reservoir tank. The only way a manual toilet can safely be flushed with fresh water from onboard the boat is to add a another tank, dedicated to the toilet only, and have the toilet draw its rinse water from that tank. You'll have to refill the tank when it empties.
It is best, when replacing a head, to do some research BEFORE purchasing the replacement, to be absolutely sure you are buying what you want, and what you need. Don't simply let the cost be the sole determining factor; do your homework before you make your selection.
And don't buy the idea that your toilet will do both - flush with raw water and/or pressurized fresh water. There IS such an animal, but it is a special order item from the manufacturer, at a "special cost." West Marine, Defender and the other discount houses WILL NOT have it in their catalog - in spite of what their unknowing but well-meaning salespeople may tell you. I personally discourage buying them, because the installation is overly complicated, and troubleshooting...hah! Should you develop problems at some point down the road, trying to troubleshoot the head becomes a real nightmare!
