This dataset shows the eelgrass data extent identified from the acoustic surveys completed within and adjacent to the boundaries of Cape Cod National Seashore (CaCo) from Duck Harbor to Jeremy Point in Cape Cod Bay, and Wellfleet Harbor from Wellfleet to Eastham, Massachusetts, August 10 – December 12, 2015, and July 12 – September 7, 2016. Full coverage sidescan and partial coverage bathymetry data were collected using an Edgetech 6205 Phase-Measuring Sidecan Sonar. In total, 5723.8 hectares of sidescan imagery were mapped at primarily 20 – 40 meter line spacing and 50 meter swath range to allow overlap of lines and a total of 19.3 hectares were identified as Eelgrass. The minimum mapping unit for this study site for eelgrass was 100 square meters and was hand digitized from individual sidescan sonar files at a visual extent of 1:1000 m.