In this tutorial I'll be teaching you a very easy way of randomly generating maps in Photoshop. First off, we'll start by creating a new project.
Great, now, before we proceed we have to make sure that our foreground and background colors are set to black and white.
Once you've got your colors all set up we'll start using our filters. Go to Filter>Render>Clouds.
It should look something like this.
Now, to get the map effect go to Filter>Sketch>Note Paper.
A new window should pop up, it should look like this. Play around with the settings, but something I recommend is setting the Graininess to 0. Moving the Image Balance will result in how much of your canvas you want filled up with land.
Click OK. As you can see, I've edited the picture and removed unwanted pieces of land by using the Eyedropper tool (E), choosing the grayish color as a foreground color, and then painting over unwanted land with a small hard brush (B).
After that you can add textures and borders and what not to stylize your map.
And done! It's a very simple process, you can use this for concept threads, fiction stories, etc. Enjoy!