Here are a few choices from cruisers we've met.
Earning as they go:
Chef, busboy, waiter, etc. in food industry in busier tourist destinations.
Steward on ferries and day-charter cruises for tourists.
Skills that cruising boats need and will pay for: shipwright, refrigeration expert, electrician, diesel mechanic.
Cruising writer, photographer. We met a young couple who were established writers who sold their scuba diving articles and photos to support their cruising. Both were in their 20s.
Donald Street and Phil Doyle wrote cruising guides of the Caribbean. Don's primary means of earning a living, though, was selling Imray Iolare charts to chandleries, and also (I believe) selling insurance. Doyle had a large schooner and took out day charters. A friend of ours lived on his trimaran in the Caribbean and sold Hawaiian Tropic products throughout the caribbean.
Our friend Shel was a merchant mariner, and he would sign onto ships to replenish his finances, then take off for six months to two years cruising, then sign on again when he needed more money again.
Many cruisers make extra money working as delivery skippers. This of course only works for experienced sailors.
Then there are the people who made their money on land and retired to go cruising. I'm not an investment adviser, but for many of the cruisers we met from various countries, investments in real estate (rental properties) or various businesses provided the income for them to cruise.
There is a huge industry, particularly in the Caribbean and the Med, of professional crew for megayachts, and I understand that the money is quite good. I just picked up a new magazine called "CrewLife" that is focused on this market. http://www.crew-life.com/
I think I've only scratched the surface.