* By coastline: The coastline is the line that separates the land from the ocean. This is the most common way to define the boundaries of the ocean, and it is used by most countries for legal purposes.
* By continental shelf: The continental shelf is the gently sloping underwater area that extends from the coastline to the edge of the continental slope. This is where most of the world's marine life is found.
* By abyssal plain: The abyssal plain is the flat, deep part of the ocean floor. This is where the deepest parts of the ocean are found, including the Mariana Trench.
Ultimately, the definition of where the ocean starts and stops is a matter of semantics. There is no right or wrong answer, and it depends on the context in which the question is being asked.
