There is usually a water supply line that runs into the bottom part of the hand pump assembly on the head. Follow this line, and it should go to a seacock. I don't know the E-25 well enough to tell you were this is exactly, but on my prior E-27 and my current Independence 31, this line went through a hole in the bottom of the vanity cabinet with the thru-hull accessed by the door in the cabinet.
Regarding flooding, probably the biggest issue is that the head is often right at water line height, and if there is a problem with the valve in the pump mechanism, or it is left in the flush position, you can end up having water just flow right in via the flush intake water line. If you look at the hand pump, there is usually a knob or lever to switch from a flushing action to a pump dry. Once you've flushed, you've primed the whole line full of water from the pump out to the seacock. If the valve at the head is left open, or leaks, you can siphon water in via this line if the head is below the water line, or you heel and it suddenly goes below etc. For this reason, I always keep the water intake seacock closed unless I am actually flushing.
There are a variety of ways you can help prevent this beyond keeping the thru-hull closed. The most common is a siphon break/vented loop. I'll let others descibe some of their installations. In fact, try doing a search on this board, and I think you'll find quite a few discussions on different ways to plumb this.
Anyway, get out your flash light, roll up your sleaves, start following hoses, poking in bilges, pull up all the cushions and look in all the stowage areas and get to know your boat inside out. You'll be amazed what you learn, and it will prepare you with knowledge to make your sailing safer.
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