Fresh Water Flushing
Only if it is a non-pressurized, dedicated tank specifically for that purpose. You can't use manual heads on pressurized systems or you'll have a strange-looking fountain in your boat's bathroom. They aren't built to withstand pressurized water. Also, and more importantly, you never want to use the same tank that you're using for drinking water - unless the head was designed and built for that particular application. These days, many upscale marine toilets are designed and built to be operated with pressurized fresh water, and are safe for that application. But jerry-rigging a marine toilet to work off of your pressurized fresh water system, if it wasn't designed and built specifically for that purpose, can result in contamination of your drinking water and the possibility of contracting a disease.