There is another recent report here on Salt Pond & Nauset Marsh, so I won't repeat any of the glowing reviews there, except to say that this is a wonderful trip with wildlife galore. The only additions I will make are:
This is a great paddle and the kids even loved playing with the hermit and horseshoe crabs at the launch, which is right off Rt. 6 just south of the Salt Pond visitors center, which is worth a visit while you are there.
The launch area has limited parking, you can drop your boat(s), park at the Salt Pond visitor's center and use their trails to walk back to the launch, but get there early or late in the day and there is generally parking at the site. There was a not so clean Jiffy John at the launch area, but there are very clean bathrooms and water fountains at the visitor's center, a short walk/drive away.
Check local maps. The launch is not well marked, but is right off Rt. 6 and pretty easy to find. If you are heading north and get to the Salt Pond visitors center, you need to go back less than a quarter mile to the launch.
If you are going to explore the marsh, you will want a pretty good map, or chart, to keep your bearing. After a few turns in a marsh, it all looks the same. However, there are some good landmarks here.