Our marine laboratory has a full-time aquarist. He worked at a zoo before we hired him. There are paid aquarists at every aquarium, as well as curators. The only problem is that it can be hard to make a living as an aquarist.
As for wildlife rehabilitation, the jobs are severely limited. A number of my fellow students work with rehabilitated sea turtles at Mote Marine Laboratory, but they receive no pay. They also do manatee and dolphin training for free.
"Research" is probably the most promising field of work. First of all, you can do research in whatever aspect of biology you are interested in. For instance, you can do fish neurobiology, or endocrinology in shrimp, or even go fishing to study the populations of sharks! It allows you to hone your interest and become the expert in one area.
You could also become a laboratory technician. That's pretty cool. You mix chemicals, perform surgical procedures, and do lots of microscopy. However you work for a scientist.