extract lake outline in google maps

I have some bathymetric data for a lake, shown here in google maps:

http://ift.tt/1vF79sB

The problem is that the bathymetric data that I have does not include the outline of the lake i.e. the boundaries where the depth = 0. Therefore, when I plot the bathymetric data, the lake appears much smaller than it actually is. Is there anyway that one can obtain information on the lake outline in google maps. For example, it would be great if I could have the latitude and longitude of the entire lake outline. Is this at all possible. I could go around the lake with a cursor in google maps and manually extract the locations, but there must be an easier way?