I would use the shape blender: copy one of your lakes, paste in front and resize, change the colors to match the blue water and beige-y map color you want and use the blend tool to get the gradient effect from the water to the map color. Creating a gradient could work, too, but it would take longer I would think. Then again i’m still using CS3.
Reply back if you want a step by step and I can make up samples.
Jonathan Ziegler
https://www.electrictiger.com/
520-360-8293