I posted this tutorial in my Hmmm map a couple of days ago already, but I figured I may as well throw it on here as well.

The tutorial describes how you can make simple dotted roads and sea lanes in Photoshop 7.0 using the pen tool. I don't know if this will work the same in later versions of Photoshop.

Here's the link: http://flaterectomy.penguihost.com/sea_lane_tutorial/