Oh how I wish it were so easy....
There is always the chance that you may find something, somewhere. I would never say it's impossible, but it is very difficult unless your skills are fresh, you have gear and/or some unique skill like languages. Here in Khao Lak a few of the shops are looking for people with German or Scandinavian languages.
As Steven said, in most of the more developed areas of Thailand you'll need to go through legal channels. And over the past few years Thailand has really made efforts to move from being a destination where you can stay for a few months (or years) and pick up side work. There ARE still places you can slip through, but it is less and less of an option.
Just as much - the wages vary wildly. One of our Instructors with 4 languages and 2000+ dives was given 400 baht for a days work on Koh Tao just a month ago. Whereas a snorkel guide in this area can easily make 1000baht per day in Khao Lak.