Working with marine mammals
Probably the topic most often asked about within marine biology is research involving marine mammals, including cetaceans (whales and dolphins) and pinnipeds (sea lions, seals and walruses). The reality is that research jobs involving marine mammals are extremely hard to come by for a number of reasons, including the popularity of the field, the fact that working with marine mammals is highly regulated (most research is done using tissue samples of sick, stranded or dead animals and not on live, healthy animals) and because funding is very competitive.
Two popular fields of research involving marine mammals are bioacoustics and vocalization (the study of marine mammal sounds), and population dynamics (studying marine mammalian behaviors and responses to environmental conditions as they impact population). As for nonresearch employment options involving marine mammals, most positions would exist at aquaria, museums and national and international conservation groups, though these are also highly competitive.