A mariner is someone who works on a boat or ship. A lobsterman who works alone on a small boat is a mariner, and so is the captain of an enormous cargo ship.
Mariner is a somewhat old-fashioned word for a sailor or seafarer, a person who spends most of their working life on some kind of ocean vessel. Today, you're more likely to read about mariners in an old book than to meet people who describes themselves this way. Mariner comes from the Latin word marinus, "of the sea," and its root, mare, "the sea."