It is often a case where you might not have the water polygon shapefile unavailable. Shape files are very important to create grids or create contour or analyse the different sets of data. In EFDC also, for preparing the grid, shapefile is required.
The method I suggest is to create a path in google earth. First of all zoom to the location where you want to create the shape file. If you are not interested in atributes, then you should choose the path in the google earth as shown below.
The next step is to go to the view tab on the new path properties.
Then you click on the snapshot current view and then it would change the parameters that are in the latitude, longitude, range etc and you cursor would change into cross hair.
The next step is to create a path and then save that path to *.kmz or *.kml
Then you to this website. http://www.zonums.com/online/kml2shp.php
This would help you to change the google *.kmz file to shape file and the shape file thus transformed is then ready to use for your project.
I hope this information helps you. If you think I missed anything or need to add anything so other people can avail of it please give your comments.