I’ve been on many Earthwatch projects, but not as a photographer. But these are more like vacations where you will be working instead of sightseeing. It’s still an incredible experience and a great way to get closer to the location, rather than as a tourist. They aren’t long trips and they aren’t cheap, though. And you could probably get more in-depth experience for less via Idealist.org.
If I hear of anything else, I’ll post it here for you.