Unfortunately winter flounder are a rarity in L.I. sound nowadays. I would think catching one w/o bait even more so. On a fly ..unheard of.
Summer flounder (fluke) are another story.
In late summer and fall when the peanut bunker are near shore the fluke will follow them in and be right in the wash. With a clouser cast from shore you will do better than a bait fisherman drifting in deep water.