If you're trying to get to THAT part of the beach and don't feel like walking through the marsh, you can take the main entrance and just walk to the left around the bend for about 30 min.
Great beach! Enter by the port o Pottys for the girls area (inc. gay parent families) keep going down further to the left for the guys... Families and stuff tend to gravitate to the right more.
You can park your vehicle in the Herring Cove Beach parking lot and walk down to the beach. The summer 2010 parking fee is $15 per day. On nice weekends and holidays the lot fills up quickly.
Beautiful beach, take a walk to the lighthouse admiring the dunes. Pay for the National Park access and get access to all the Cape Codparks for the day.