charter business for a 50 ft Motor yacht

I'm going to back up to the most fundamental question, if you're only going to use a boat once a year or so, then why own one? It would be cheaper and less trouble for you to be on the other side of the charter and charter once a year and you could do it anywhere in the world.

You're highly unlikely to ever make money chartering unless you have an incredibly good situation and you've got things to deal with. Owning boats isn't for everyone, not even those who love boating. If I only had two weeks a year to boat, I'd charter one year in Florida, one year in Washington, one year in the Virgin Islands and on and on like that. By the time I could use a boat more, I'd know what I wanted and where. If I then wanted to put it in charter, I'd now have some of those contacts too.

I've been lucky, I lived near and then on the lake and now live on the coast. In my worst year of us, I used my boat at least 60 times. Ownership made sense. But today on the lake I lived near, there are plenty of rentals and boat clubs and they're perfect for the once a year boater.

Although I agree with you and would certainly have a for sale sign on a boat I used that little there could be good reasons to keep it. There's the 2nd home tax advantage, a place to park money, write-offs, bragging rights, as well as having a presence in the Hamptons which may bring the OP business and social opportunities. Of course if looking to make perfect sense out of owning a boat I doubt anyone would own one except for business. Big holes in the water in which to dump large amounts of money. The fact is that most boats are bought on a dream, and then become dock queens when the realities of life rear their heads.

I'm always the dark cloud, but is a decent marina going to allow you to run a BnB type charter operation? Dunno. Constant change of non nautical minded clientel, parties, toys, etc. How would the resident vessel owners react? Particularly the owners berthed near the venture.

Have you seen the "resident vessel owners"? Not exactly sedate, and they're not stopped from having "non-nautical minded" friends aboard. I used to run a couple of (not rented) boats right there that routinely had 15 or 20 heavy duty partiers aboard from Friday through Sunday night. Most people viewed them as an amusement if they didn't join the party. I wouldn't be too concerned with a family who can afford Hampton's rates. However I did suggest clearing it with the dockmaster and especially the town. What exactly is the difference between an owner who has friends aboard or loans his boat to friends and one who rents it? It's still just 4 to 6 people sleeping aboard and they're subject to the same marina rules as everyone else.
Now if someone bought a bunch of slum boats and turned them into year-round low income housing that would be a different story, but there's not much chance of that happening given dock rates on LI, although I do know of a couple of up-island marinas that did that themselves.

Depending on the marina, you're are correct -there may be many resident "party" boats. But the OP is talking of a boat type BnB. You may not have had experience with BnB's on land. Most municipalities have heavy regulations on them because of the enormous nuisance factor they invite .... If you think the Hampton visitor is always a sedate family of 4 -6 people just sleeping aboard you haven't spent much time in the Hamptons/Montauk area either. I only asked the question, will a high end marina permit it? I dunno, as I said. Maybe they all will. But you gotta know that for your business modeling.

Hampton's visitors sedate? ROFLMAO!!! Like 20 drunk naked people dancing as the boat pulls into the dock? That's owners. I'd expect more from someone paying a couple thousand per night for the weekend.
That's why i told him to check with the town. I know my town requires an inspection and permit for rentals, but that's to prevent slums. Summer rentals are big business and expensive in that town. And as I said they do it with RVs (even in my town). A boat isn't all that much of a different situation.

I am not at all trying to discourage the OP but boats are not houses in that they are way more complex. My sister and her husband stay on board without me a couple of times a year and I usually tell the dockmaster or whoever is looking after the boat for me that I will pay them to help her out with what she can’t figure out or what she messes up. Typically having the AC shutoff because she didn’t turn on the AC pump breaker. Running the freshwater tank dry because they don’t fill the water tank and messing that pump up. Or I can See somebody putting water in a diesel tank. She can’t put out the sea stairs by herself (her husband is a very good person but a handyman he is not). In other words getting her on and off the boat can be an issue and she has been on the boat and see how we do things. I can’t imagine the support non boaters will need for a weekend. I realize this is maybe specific to me but my boat is a very personal belonging and I couldn’t stand the thought of a bunch of drunk non boaters on board screwing stuff up. To each their own but not for me in the least.

Many marinas do not allow BnB operations. Others may allow but only under a commercial lease agreement and then even require a crew member to be on board for obvious safety reasons.

considering how regulatory loving NY can be, I’d be surprised if it is allowed especially in a high end place like Sag

You're absolutely correct, but it's the same for any vacation rental. You have to have somebody on call to fix things fast whether the rental is a house or a boat. Like with any charter, and especially a bare boat charter you have to explain the systems to them, especially the head. Water is turned on dockside at the start of the weekend, so there's no tanks to be filled. If you think about it what do you have on a boat that doesn't move that you don't in a house other than the systems being more finicky?

I know there's boats there that charter, including some rather large ones. In fact I wouldn't be surprised if some do this exact thing (with or without town and marina permission), but again that's why I said to check with the marina as well as the town because I always recommend operating on the up and up. One thing I learned early is that the answer is always "No" if you don't ask.