I would think you'd have to be independently wealthy, or financed by someone who is, to be a full time treasure hunter.
My idea is, shipwreck salvage would be the most rewarding. The oceans cover 70%+ of the earth's surface and man has navigated the oceans since the start of recorded history. Anything you do is going to be hard work, require quite a bit of outside influence and assistance and perhaps 1 out of 100 times, you'll find something of real value (not nec. monetary, mind you) to someone that might put you back to square one and allow you to break even.
Notice I said
rewarding, not profitable...
Things I keep in mind along the vein of operating costs:
lawyer retainers
divers, medical staff, pilots, ship captains, translators.
Political advisors, researchers
Insurance and bonds
Relating to anything you'd find - it's value is based on only three things:
How much it's worth to someone willing to pay for a luxury item (assuming you're after gold or artifacts)
How important the item is from a historical point of view, and then usually it's desired by a person or collective body that cannot pay for it's monetary value (i.e. a historical society or museum)
Is it worth more to you on your mantle for prosperity or on the auction block for a profit to recoup your operating cost (see bullet 1)
If you're a guy or gal with a SCUBA qual you could look for things in shallow water and break even finding jewelry. If you had a full blown operation (ala Ballard) you could find historical wrecks or commercial wrecks and salvage them for the company who either lost the ship or is interested in its recovery.
Again, become independantly weathly or find someone who is, and share their passion.
I'm the first to admit I love a good daydream or fantasy, but when I get down to it, I try to remain realistic even if that's not fun.
If I'm totally off base here, please - set me straight.