[Literide]

Montauk Yacht Club is not a traditional yacht club, it is a resort and spa open to the public.
Formal tradionally means white tie and tails, and is rarely seen at even traditional yacht clubs.
I would check with the host but definitly avoid whites and footwear is key. On board or on land?
I suspect blazer and tie with darker slack for the season would be fine but you really need to check the hosts definition of formal.