That would definately rule out Hingham as well. You have to trudge through knee deep mud in order to get to open water.
At least in Duxbury, you don't go through mud and you can get into the deep water channels fairly quickly. Overall I'd say the Herring River might be the best bet since you put in right in deep enough water. And us paddlers can catch the incoming tide on the way back to the ramp. Not too far a paddle/peddle to the North River, South River and the mouth if we want to go out in front of 4th Cliff.
Maybe someone can come up with a better location, but that doesn't sound too bad to me.
