I know this is weeks old, but in case anyone searches this question later, I'll answer.
The odds of you getting a seat to ACK or MVY in summer are infinitesimal. Until the season really winds down in mid September or so, there really aren't any ever any open seats. September and October are the best times to visit anyway, most places are still open but it's just the real Islanders left.
As far as dress code, I'm sure it's officially bus casual, but I can't think of anyone who would care; you may not even get a chance to do the "required pleasantries".