Save the Oceans, the Nature Conservancy, I like. I am involved in fund raising for Reefcheck. We lay transect lines and count indicator fish for a baseline and ongoing data reflecting the state of a given reef. The data all goes to United Nations and is used to make decisions about global warming, etc. I am more in the fund raising side, parties, events, etc. A big one coming up as a matter of fact....
REEF is another good (and fun) one, You basically tally fish on these waterproof preprinted "scorecards" specific to the area you dive. I enjoy doing those very much. Those little golf pencils work great.
Whatever interests you, it is great to get involved. You can check the financial statements on a lot of these groups to see where the money goes...
Definitely check out Mr. X on scubaboard. He always has some good ant-whaling petitions and suggestions (like boycotting Gorden's ).
Aloha
PS are those prairie dogs in your Avatar?? I took a lot of prairie dog photos at Geronimo's grave this summer!! So sorry, I am easily distracted, but I had never seen them before! they talk a lot!!