Both floating and rooted plants grow in the ocean, but single-celled floating phytoplankton are the most common. Most of the ocean floor is covered in darkness, so rooted plants do not grow there.
Most of the world's oceans are filled with plant life, which varies from single-celled organisms up to complex rooting plants. Basically, there are just two types of plants in the ocean, plants with roots and plants that float freely in the water. Since, the ocean is so deep, much of its floor is far below depth that sunlight can breach. This means that rooting plants are confined to shallow areas, usually close to the coasts.
Phytoplankton is the most common type of plant life found in the ocean. However, it is so small that it cannot usually be seen by the human eye. A microscope is needed to see the millions of diatoms that are present in a single bucket of water. Phytoplankton floats freely in the ocean using water, nutrients and sunlight to create their own energy. A similar plant is the multicellular algae seaweed, which can be found in a wide variety of species. Some seaweeds are free floating, and others are rooted or attach themselves to surfaces like rocks.