Plants are autotrophs, meaning they produce their own food using energy from the sun. Plants can survive underwater as long as they are close enough to the surface that they still have access to enough sunlight to sustain photosynthesis.
There are two main types of plants found in the ocean: plants with roots that are attached to the ocean floor and plants that drift freely through the water. Plants that are rooted to the ocean floor are not found deeper than a few hundred feet because sunlight cannot reach that deep into the ocean. The area where these plants can grow accounts for an extremely small portion of the ocean.
The most abundant plants found in the ocean are phytoplankton. Phytoplankton are single-celled autotrophs that drift throughout the surface waters of the ocean. Phytoplankton are much too small to see with the naked eye, but large clusters may give the ocean surface a green hue. These plants need abundant sunlight and nutrients to grow. Tropical waters have the most sunlight, but lack a sufficient amount of nutrients, especially nitrogen and phosphorous. Because of this, most phytoplankton are found in cooler waters where the deeper parts of the ocean bring these essential nutrients to the surface. Phytoplankton, marine plants and algae provide a high amount of oxygen to the world. They also take in large amounts of carbon dioxide due to photosynthesis.