This data can also be found on the
USDA Geospatial Gateway. Pick your state and county of interest, then download the 12-digit watershed boundary data set under the "Hydrologic Units" section. It's by county though, so other bodies of water are included as well. Besides having multiple bodies of water in the same file, an additional issue I ran into was the data for my lake had both shoreline boundary and island data in one file. This confused RM and the results were messed up.
To get around this, I loaded the shapefile in QGIS, exported to KML, opened the KML in Google Earth and zoomed into the body of water of interest. I right clicked on the shoreline boundary and selected properties. The associated entry (out of hundreds in that county's folder entry) is then highlighted in the left menu. I then closed the properties window, right clicked on the highlighted entry in the left menu and selected "Save As" and saved to KML.
Using notepad++, I then edited the KML directly. The longest set of points was the boundary, which I copied to a separate polygon KML file. Then I took all the island boundaries and combined them into another separate "folder of polygons" KML file. I had to edit the KML code a bit, as the islands were originally designated as "inner boundaries", and they had to be changed to polygons. I used the "Search" and "Replace All" functions in notepad++ to do this fairly easily.
Then in RM, load the boundary file first, go back into the edit menu, and set it to "boundary", then import the islands file. My island file was 97 boundaries, and RM took about 15 seconds to load them all. The default setting is "island" in RM, so I didn't have to make any further changes under the "Edit" menu.