In the case of both of these positions, the word "marine" means connected with the oceans. In both cases, the biologist and the conservationist are working with plants and animals that live in, on, and under the water.
Biology means "the scientific study of all forms of life, including plants, animals, and microorganisms." A marine biologist specializes in studying life forms in oceans, lakes, rivers, and other bodies of water.
A marine conservationist goes beyond studying the plants and animals in their watery environment. The conservationist looks at how that environment might be endangered by pollution, examines how the life forms are being impacted by climate change and the rising temperature of the water in which they live, and monitors other changes in factors that affect the water and its inhabitants, both plant and animal.