Vegetation relies on photosynthesis for energy; sunlight can't penetrate the ocean depths, so plants can't grow in deeper waters. However, shallow coastal waters are a different story. Many varieties of sea vegetation thrive to depths to about 600 feet (183 meters) in the so-called “euphotic zone.”

Although you'll find many different types of "plants" in this zone, few of them actually live on the ocean floor. Seaweeds, which are actually algae, may anchor themselves to rocks on the ocean floor, but they live close to the surface. The list of underwater flora, or plants that live in the ocean, isn't long. It consists mainly of various types of seagrasses, and it arguably includes mangroves, which grow in shallow water in the tropics.

When you think of a terrestrial plant, you visualize roots and a vascular system that transfer nutrients from the soil to the leaves and flowers. Seaweeds have neither roots nor a vascular system. Giant kelp, which belong to the class Phaeophyta, or the brown algae, anchor themselves to rocks with rootlike structures called holdfasts. They grow quickly – as much as 2 feet a day – to float near the water's surface, where sunlight is more available. Similar brown algae include rockweed and Sargassum, which is common near coral reefs.

Seaweeds also include red algae (Rhodophyta), which include Irish moss and dulse (Palmaria palmata), which are important in different cuisines. These may anchor themselves to rocks or float freely. Green algae (Chlorophyta) is a diverse third class of algae that includes 700 species, the best known being sea lettuce (Codium spp.). All seaweeds, like true plants, contain chlorophyl for photosynthesis, but green algae, unlike the other two classes of seaweeds, contain no pigmentation to hide the characteristic green color of the compound.

Unlike seaweeds, seagrasses do actually root themselves in the soil at the bottom of the ocean floor, and they have leaves and flowers, just like terrestrial plants. There are four different groups: Zosteraceae, Hydrocharitaceae, Posidoniaceae and Cymodoceaceae, representing 72 different species. The name of the species is often based on its appearance, such as eel grass, tape grass and spoon grass. Turtle grass is one species that is so named because it is a favorite spawning ground for sea turtles.

Seagrass is often characterized as the "lungs of the ocean" because of its capacity for absorbing carbon dioxide and generating oxygen. One square meter of seagrass can generate 10 liters of oxygen every day. The seagrasses function as habitats for various forms of marine life, including crabs and other crustaceans, sea mammals, mollusks, worms and many others. Seagrasses tend to live in shallow water about 3 to 9 feet (1 to 3 meters) deep, but some can grow at depths of 190 feet (58 meters).

Mangroves are trees that grow in intertidal waters in the tropics from 32 degrees North latitude to 38 degrees South. The don't actually grow underwater, but their roots are immersed by saltwater, and they contain a special salt filtration system to cope with it. A mangrove swamp is known as a mangal, and it constitutes a distinct biome of its own, Mangroves can't get oxygen from the soil, so they have to extract it from the air. Despite this, scientists have determined that a mangal is an excellent carbon sink, which means it has a high capacity to absorb carbon dioxide.

Sea grapes (Caulerpa lentillifera) is an edible green algae that thrives in the vicinity of mangrove swamps. This succulent algae, sometimes called "green caviar," is a favorite menu item in many Asian countries, including the Philippines and Japan.