This is aimed more at SCUBA, but it may whet your appetite;
On the freediving front the most common way to earn a living seems to be as a freediving instructor or underwater photographer. If you progress into spearfishing or other underwater harvesting (scallops, lobster etc.) there may be opportunities to sell your catch, depending on the laws in your area. There are many more novel ways to earn some cash from your skills, its a case of using some ingenuity and seeking out opportunities.
And of course its always worth taking a trawl through the "Jobs offered" thread on here;
Above all remember to stay safe, make sure you dive with a competent buddy. No payday is worth risking your life for. If you can, take a course, the skills you learn will not only help you stay safe, but also help towards any aquatic career you find, and the certificate demonstrates your competancy to any potential employers/clients.