Don't put the vent into the sink drain. It will contaminate the tank.
Some boats vent to the sink like shown below (older Swan). The other option is high in the boat near the tanks, ideally on center. When filling water will come out the manual water outlets at the sinks first as they are lower than the vent. Inside vents avoid salt water contamination.
Living aboard in Victoria Harbour
Last edited by mitiempo; 10-18-2010 at 12:12 AM.