Paddled Nauset Marsh in September 2010. Launched from Hemenway Road two hours before high tide.
The north end of the marsh had very little water (just a few inches in many places) even at high tide, and it was of a swampy quality with a deep muddy bottom. Paddling around the north end, past the Coast Guard Station, and then down toward Nauset Beach, I walked the last 1/4 mile before the Atlantic Ocean inlet.
At that point I was forced to return rather than spending any time at that beautiful location, to avoid getting stuck in the mud at low tide. Unless you're an exert paddler, you will find it impossible to pass over from the northeast side of the inlet to Nauset Beach on the other side of the inlet. Nor will you be able to cross the surf to access the west side of the marsh and return to the landing that way.
It seems preferable, if Nauset Beach is your destination, to paddle to the right (south) as you leave the launch. There is much more water there. I was told (could not verify myself) that there is a stream that cuts across the marsh approximately in front of the fort and goes over to Nauset Beach.
The water at the inlet is extremely rough as the surf hits the very shallow area, for experienced surfers only. You will hear and see the inlet well before you get there, in time to avoid it.
In contrast to the north end of the marsh, the central and southern parts have clear water, lots of sand, and tons of birds.
The area on either side of the inlet is truly beautiful---long sand beaches, great surf.
I saw large colonies of all kinds of big sea birds.
A great destination, but choose your route carefully. Don't get caught in the north end as the tide is going out. I assumed I had 3 hours on either side of high tide, but that was not true for the northern portion.
Large parking lot. Good launch. Portapotty (clean).
Free in the fall. Not sure about summer.
There are a couple of launches off route 6 north of the Orleans traffic circle. Hemenway Road is just after the Eastham visitor's center.