Earth is a water planet, possibly one of a few in our entire galaxy.
Presence of water on its surface is determined by the Earth's size and composition, and by it's position with respect to the Sun. To the best or our knowledge, life evolved in the oceans several billion years ago and still today, living cells of all life-forms are still bathed in salty fluids. The scientific method is just a systematic way of asking and answering questions about the world around us. Marine science applies this scientific method to the ocean and atmosphere, the solid Earth underneath, and to all living organisms dependent on it. Oceanography is not a pure science in its own right, practiced by scientists specifically and narrowly instructed in its investigative methods.