Here's the 2008 June pelagic report, 2009 didn't run due to lack of people and from what I hear this year needs a bunch of signups soon or it won't go either. I suspect the potential is much better than the results show (apparently a research trip had a Bermuda Petrel last year a bit further out).
The Cape's never bad, but might be a bit crowded and have too much traffic (weekend especially). I'd head west to the Quabbin area (1.5 hours or so west of Boston), which has lots of breeding warblers and stuff.