If it has to do with the ocean or sea, you can call it marine. Your friend in US Armed Services who is a Marine probably spends a lot of time near the ocean.
Marine life is everything that lives in the water: plants, fish, sharks, algae, crabs, seahorses, giant squid, and the list goes on and on. When you study things that live in or near the sea, you are either a marine biologist or just really excited about ocean life. If you work in a field that deals with the ocean, boats, sailors or the navy you could be, for instance, a marine meteorologist.