Boston out to the Cape to fish Brewster or Monomoy Flats is going to be about an hour and a half, assuming no traffic. Monomoy requires a ferry boat to reach (http://monomoyislandferry.com/
) as its an island. You need to book this ferry well in advance as it will sell out oftentimes (not just fishermen - bird watchers, shell collectors, photographers, and everyone else will use it to get out there).
Also, should you be interested in this, you should check the tidal chart for the day before you commit. The ferry run, maybe, 8am drop-off and 3pm pickup, or something like that (cant remember, been a while). High tide in the morning/middle of the day is not going to be good. Low tide morning, mid afternoon will be better. The real estate for fishing is rather limited when the tide is in.
Also, be careful when the tide is coming in. That shallow cut you crossed to what looked like dry land might require a swim in an hour, not to mention that dry land may be under water in two. Ive done some (unintentional) swimming on account of losing track of time/tides, and let me tell you its a bit nerve racking going for a swim, especially when there are seals all around them, and , with them, Mr. Great White lurking not too far away (http://www.mass.gov/dfwele/dmf/spotl...09.htm#gallery
Its a cool area, neat place to fish. I know there are abundant guide outfits out on the Cape, though I have no recommendations. Im sure you can find something if you look, or someone else can recommend someone.