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The dockside pump-out is a vacuum pump Frank, you do need to see that it seals fairly well in the fitting on your deck, the nozzle is usually a tapered rubber thing that seals pretty good.
Also, remove that cap at your dock first and be prepared to chisel it out and replace, they are often corroded (sp?) in place.
Thru hulls for flush water and macerator pump closed so you don''t suck seawater up thru tank from bottom.
Most boats have a "macerator" pump at the tank so you can pump the tank out into the ocean beyond the 3 mile limit, the pump grinds everything to size required. Some boats will have y-valve so waste can be directed to tank or direct to ocean. In some areas these must be fixed to direct to tank for environmental reasons.