Furnished rentals in this part of the south shore are few and far between. Adding to that a seasonal rental pushes it further down the funnel of rental options. If you let me know what kind of space you're needing, there might be an unusual situation that possibly could work for you. It is not, however, waterfront.
A second possible solution could be getting an unfurnished rental and getting rental furniture for the period of time you need it. I deal with these companies all the time as a real estate agent/stager and you can wind up with a great look.
Another furnished short term rental might be one of the residence hotels on Long Island. If you need more detailed information, just let me know.
Whatever you opt for, I wish you luck.