After doing this trip two years in a row, the following seems to be true, and some of it is obvious....
Whale watching boats can be very birder-friendly, if you pre-negotiate with the key people, have a lot of paying birders on-board, and discuss your birding goals before the trip (including a photograph of your #1 target bird). And the DolphinFleet is excellent and highly recommended.
Usually seabirding from a boat (2012) is much better than land-based sea-watches (all day 2011 and several hours in 2012), and enables much closer viewing of the birds (seabirds usually keep a distance from the beach).
The more experienced seabirders on the trip, the more good birds you will see.... obviously. Partly because you need multiple experienced seabirders scanning the horizon in every direction, with coverage from the front, back, and top of the boat. We used walkie-talkies to keep the back and top/front coordinated.
You do not have to go many miles offshore to see lots of shearwaters and jaegers, especially around Race Point, MA in late September. One and two-day trips out to the canyons along the shelf break are great, but many of those same seabird species can also be found within five or ten miles off Race Point in September.
The tail end of September seems to be a very good time to find seabirds around Provincetown, but for 2013 we may want to try several weeks earlier, TBD. See the calendar of seabirds at Cape Cod at...
Cape Cod & Stellwagen Bank SeaBirding: Calendar of Seabirds at Cape Cod
