Thank you all for your advice. It's helpful. I'm thinking of two different options. 1. Shove off from Salem at last light on July 19th and sail overnight to P-Town. Then rest and head towards the canal or rest and explore P Town and overnight there.
Or head out in the morning on July 20, explore Boston Harbor (which I can do whenever, but I want to get a feel for it before I take people there) then head to P-town during the day for an overnight. Then hit the canal in the afternoon the next day. It looks like 19nm to the canal from P Town and I'm guessing about an hour to get out of the harbor so I'd leave five hours to arrive just after the tide has turned.
I'm leaning towards option 1. Comments and criticism are welcomed.