Boat-Based Business

I find it interesting, that the only time you really hear about how cruisers make money underway is after the first book. Most of us can sand, paint, even perform basic engine repairs for other cruisers, but how can someone make a "living" while cruising. I have had no problem while cruising within the US, but what about Mexico, or Europe? Where can you run a business from your boat without violating local laws, and what are the limits?
Are there any traditional trades that are more marketable than others? Are there any that lend themselves favorably to the nomadic lifestyle of the cruiser?