I have a full-time job with a promotional marketing company that I'm fortunate enough to still be able to do from the boat, but I also have side jobs that I do for extra income.
I've updated websites for people (found on Craigslist, word of mouth, etc.) as well as built some from scratch. Simple stuff, as I'm not a professional web designer, but I know enough to make a decent looking site and I can read/write html.
I also make jewelry and have my own online jewelry shop called Maggie & Milly. You can check it out here! www.maggieandmilly.com
I write a blog called Saving To Sail
that has a post on ways to replenish the cruising kitty
Basically you just have to be creative and versatile. Some things may bring you money one month, but not the next, so just take that into consideration. Some months I make a nice chunk of change making and selling jewelry, but sometimes I thank my lucky stars that I still have a full-time job.