I'm looking to add color to a map but get an error message that it has too many points. For example, I'm trying to apply a gradient mesh. Or I've tried recoloring as a Quick Action. No avail. Any ideas on how to create coloring as shown in this preview?
Of course, but as per the warning messages you'd have to reduce the number of anchor points in order to use the desired functionality. Often that is possible with Illustrator's Simplify command while retaining the overall shape/appearance.
That's what I would try first.
As the threshold for creating grandient meshes is max. 100 anchor points per object, you may also consider to create separate gradient meshes with simplified paths, put them behind the original paths that define the lake boundaries and use both to create clipping masks or opacity masks.