Depending on your license, I believe there is a geoprocessing tool that can do this for you. How are you extracting the saltmarsh area?
Try Conversion Tools\From Raster\Raster to Polygon tool, use Simplify Polygons, then Dissolve them.
Dissolve is used to combine adjacent polygons into one polygon (it "dissolves" the internal lines between them). Looks like your output may already be dissolved. Dunno why it pixelated that badly. I've never seen polygon output that rough.