Gino, Nikoli,
The Montauk Harborside is right near the East Hampton Town Ramp. I'm sure that's the "neighboring boat ramp" and technically, you do need a town permit to launch there (this year I believe they went up to $200 for non-residents). Maybe as long as you launch and park your vehicle at the motel, you won't get bothered, but they do ticket down there. I got one in December when we were out there duck hunting.
Nikoli,
As long as it's calm, you can fish pretty much anywhere in the daytime out there. Most of the rips aren't that bad that you'll get thrown around too much. They use flats boats in the Montauk Guide House show, and they're about the same size with no freeboard. At night, if you have light SW winds, you'd be fine fishing along the North side. The land protects you from the wind and the ocean swells.
Plenty of fishing to do out there in a 17' boat.
Chris