What is the difference between aquatic and marine life? One is a subset of the other. All marine life is aquatic but all aquatic life is not marine. Perhaps it is best to start with defining the terms! Let's check out the definitions given by a single source, The Babylon Dictionary.
Aquatic life is defined by them as "All forms of living things found in water, ranging from bacteria to fish and rooted plants. Insect larva and zooplankton are also included." This means anything that goes in the water during part or all of i's life cycle can be considered aquatic life. It also means any type of water from stagnant pools to the depths of the Pacific Ocean. There are a lot of different types of aquatic systems from the center of bromilliads in the canopy of the rain forests to the great oceans that cover over 70% of the earth's surface.
Marine life, as defined by the same people, is "Plants and animals of the sea, from the high-tide mark along the shore (also see Shore Life) to the depths of the ocean. These organisms fall into three major groups: (1) Benthos: plants such as kelp and animals such as brittle stars that live on or depend on the bottom; (2) Nekton: swimming animals such as fishes and whales that move independently of water currents; and (3) Plankton: various small to microscopic organisms that are carried along by the currents." That definition is self explanatory.
So basically aquatic life refers to anything in the water. This can mean tadpoles and mosquito larvae living in hollow trees (or those rain forest bromilliads) filled with water up to the Great Blue Whales that cruise "the seven seas". Marine life is only found in saltwater and must live below the high-tide mark. This means that all marine life is definitely aquatic but all aquatic life is not marine! The difference between aquatic and marine life is that something which requires water is aquatic, but not necessarily marine.
This does raise some questions. Are marine turtles classed as marine life since they build their nests and hatch above the high-tide mark? And where do marine penguins fit in? What about those crabs that run in and out of the waves along the shore? This will bear further investigation!