[Award Winner] Technique for adding coastal detail as you zoom in.

[RPMiller]

Threshold is under Image-->Adjustments-->Threshold...

As for turbulent PS doesn't have an option. You'll just have to keep rendering clouds until you see something you like.

[Ascension]

What I do if I want to zoom in on an area for a larger map is I make the main landmass white and put it onto a black layer. Then filter > pixelate somewhere between 3 and 12 (this puts a bunch of white, gray, and black jaggie shapes in where the white and black meet). Then Image > adjustments > brightness/contrast and turn the contrast up all the way to remove the gray. Now you have a black sea and white newly jagged landmass.