You have several problems...
The first one: either you're leaving the dry/flush lever in the "flush" position all the time...or the dry/flush valve has failed. Whichever it is, whether it's inconvenient or not, the head seacock--in fact, ALL seacocks--should always be kept closed except when actually in use. To rely on a valve that can easily fail--and often does--risks the safety of your vessel, your children and your elderly passengers. It's a LOT less inconvenient to open and close it each time than it is to put 'em all in life rafts. The second problem: Water tries to seek its own level...so when you open the head intake seacock, water rises in the intake hose to the waterline on the boat...if that's above the bottom of your toilet bowl, water will rise in the toilet bowl...if that's above the TOP of your toilet bowl, water will overflow the toilet bowl. There should be a vented loop between the pump and the bowl that's at least a foot above the boat's waterline at any angle of heel, so that water cannot rise over the loop and find its way into the bowl. However, that still does not excuse you from keeping the seacock closed except when the head is in use...especially when underway. There was a recent post that illustrated what can happen if you don't. Third problem (or, so how's the water getting into the holding tank?): the joker valve in your toilet discharge is leaking...apparently at about the same rate you're taking on water, or your bowl would fill up, faster than the tank. Joker valves wear out...so do all the other seals, valves and gaskets in marine toilets. When, if ever since you've owned the boat, was the last time the head was rebuilt? How old IS the toilet? Yes, replacing the toilet with an electric toilet that draws flush water off your onboard fresh water system is one solution to all of the above problems, but that's the wrong reason to replace the toilet. You can't replace every system on your boat to avoid responsibility for safe operation and proper preventive maintenance...and electric toilets--especially the VacuFlush--present their own set of maintenance and operation issues & problems.
