The Piermont Marsh is located in Piermont, New York, about 2.5 miles south of the Tappan Zee Bridge There are several ways to see the Marsh.
The Piermont Pier is a mile long linear park that stretches out into the Hudson River. You can walk along sections of the marsh as you reach the Hudson River. The entire marsh is visible to the south as you head out to the end of the Pier.
There is an entrance to Tallman Mountain State Park at the bottom of Ferdon Avenue. Follow the dirt road, at this entrance. It takes you alongside the marsh, and continues after you pass the pool area of the park for about another half a mile before coming to an end. There are also good views from the Long Path (blazed green), which heads from the Ferdon Avenue Entrance into the hills above the marsh.
There are several overlooks on the south side of the marsh, as well. Those are best accessed by hiking in from the small parking lot near Oak Tree Road on Route 9W. Follow the carriage road into the park and look for a few small trails heading towards the cliffs (these are unblazed trails branching to the right) right after the road curves sharply to the left just under .5 of a mile from the parking area.
By Canoe or Kayak:
There is a good unofficial put-in at the bottom of Ferdon Avenue. The Sparkill Creek heads out into the marsh for a .7 of a mile or so before entering the Hudson River. The Sparkill Creek is one of few places in the area where you can safely paddle a marsh at low tide (though the mouth can be shallow during very low tides). Most typical marsh channels will empty out as the tide gets lower and you can get stuck. There is a good side channel at the mouth of the creek which can be paddled at higher tide levels for some distance. Once you get out into the river, there are two more side channels which enter the marsh from the river. Both should be paddled at higher tide levels only. To access, paddle south along the marsh for about .6 mile to Crumkill Creek. A second channel enters at 1.3 miles from the Sparkill, just at the south end of the marsh. There is seasonal boat rental at the bottom of Ferdon Avenue, just across from Tallman Mountain State Park, at Paradise Canoe and Kayak.