It's not the most realistic or the most attractive, but it's quick. Take a look.
These particular pieces were done for Axis and Allies Minis, but could be used on a larger scale. It would probably work best for ocean, as the name of the mat implies.
I used a Lemax Ocean Display Mat. This is a clear, vinyl mat with blue waves printed on it. The bases for the lakes and marshes were cut from plasticard and then traced onto the mat. The marshes and lakes were attached to their base with white glue. The rivers were glued to a sheet of paper with white glue. These were all sandwiched between heavy books and left to try overnight.