Threemile Harbor is a well-protected bell of water on the southern portion of Gardiners Bay. Threemile Harbor is 18 miles west of Point Montauk, nine miles south of Plum Gut and just around the corner from Sag Harbor. Inside Threemile Harbor is an excellent array of some of the best marina facilities in the area, all tucked in a well-protected harbor.
Use NOAA Chart 13209.
From Gardiner Bay, the first aid to navigation you will want to find is red and white Morse (A) buoy TM about a mile north of the breakwatered entrance. Once you have cleared the red and white Morse (A) buoy, head farther south to intercept green can 1 and red nun 2.
Next, the harbor breakwaters are marked with a quick flashing green light (unnumbered) to starboard, and a quick flashing red light (unnumbered) to port. Inside the breakwaters, the channel has six foot depths, and this trend continues on down the passage for about a mile. The channel tends to favor the port side of the channel on entry, but there are no charted aids to navigation to specifically guide you in. If in doubt, follow one of the locals in. Six- to ten-foot depths open up once you arrive at the town os Threemile Harbors waterfront.
The west side of Threemile Harbor shallows to depths of two to three feet, and because of this, all of the marinas and marine facilities are situated on the eastern and southern portions of the harbor. Eleven facilities line the shores, with repair yards, yacht clubs and marinas making up the mix (all of them top-rate facilities). It is easy to find a slip for the night, and there is also an open, but good anchorage just west of the towns waterfront in 10-foot depths. Keep in mind that this is a No-Discharge Zone; all effluent must be kept in a holding tank.