Wayne -
I'm not sure he meant to vent it below the waterline. That would be quite trick. I think he meant that an open-to-the-air vent, either on deck on in the side of the hull (well above the waterline), would form a Venturi when the boat is moving. Actually, don't a lot of boats have an air intake scoop facing forward, to ram air into the interior compartments or spaces, and another similar scoop facing backward to exhaust interior air?
Anyway, long ago, two of our members built simple vents for their toilets. In each case, they took advantage of the round access hatch in the top of the toilet, opposite (and the same size) as the gauge opening. A piece of PVC pipe was fitted into the opening, and an elbow on the pipe took the pipe through the wall. It has been a long time, but I seem to recall that one of them ended the whole process there, with an open-ended pipe (and some trim) coming through the wall, which made a good venturi when moving. The other guy went a bit further and added a small 12-volt fan to the system, to make it a power vent. Both had some kind of closure on the outside.
I can find only one of the threads. If anyone finds the other, please post a link.
http://www.trailmanorowners.com/foru...ead.php?t=1012
Bill