>>3825527You could just draw it over with a very low opacity black brush. It'd be the most accurate, one of the auto ways would be playing with layer styles.
Copy/paste that blue thing to a new layer. Enter layer styles for that new layer and play with outer glow, or drop shadow. I'd do it separate for each lake, and play with masks to fine tune it.
What are you trying to accomplish in openttd with it, anyway?
