First, let's emphasize that 2/3 of the surface of the Earth is covered with water, which makes marine ecosystems the largest ones on the planet. So in terms of simple biodiversity and overall production of living organisms, marine ecosystems contribute more than any other. The World Resource Center estimates these ecosystems produce nearly 1/3 of the world's biological production.
This means that they are essential as to the production of food, for humans and all other aquatic species, as well as many types of birds and some land mammals.
In addition, many people forget, or were never aware, of the marine ecosystem's role in scrubbing the atmosphere of CO2, as the ocean absorbs Carbon Dioxide and stores it in deep water for extended periods of time as opposed to releasing it into the atmosphere.