The earliest plant life began in the water as long strands of algae, similar to seaweed found in oceans today. Migration onto land was a major advance for the plant kingdom, precipitating the growth of larger and more diverse species. With the exception of algae, today's aquatic plants moved back to the water from terrestrial environments, developing adaptations that allow them to survive the challenges of an aquatic environment.
On land, plants utilize light from the sun and gas drawn from the air around them to produce, and use energy through photosynthesis and respiration. Most plants produce pollen, often carried from flower to flower by a pollinating animal, and then seeds. All of these functions are challenged in an aquatic environment. As light passes through water, it quickly loses the wavelengths necessary for photosynthesis. Submersion also impedes access to atmospheric gases like carbon dioxide and oxygen that are necessary for photosynthesis and respiration. Other physical and mechanical challenges---such as periods of flooding or desiccation and damage caused by currents and waves---require adaptations as well.
Aquatic plants vary in how their leaves are exposed to the water. Submersed plants, like pondweed, exist completely underwater, and their leaves have developed very different adaptations than terrestrial plants to survive this environment. Leaves tend to be feathery, giving the plant several advantages. First, more surface area is available for the gas exchanges needed for photosynthesis and respiration. Also, feathery leaves are less easily torn in strong currents. Land plants possess pores on the undersides of their leaves called stomata, and these are absent in submersed plants because they do not need to exchange gases with the air. Since they are surrounded by water, they also lack the vascular tissue needed to conduct water from the roots to the leaves.
Aquatic plants like lily pads float on the surface, while horsetails emerge from the water completely. Despite their environment, the tops of floating leaves and the entirety of emergent plants tend to have a waxy layer that makes them waterproof. Because even a thin layer of water diminishes the light available for photosynthesis, water pooling on top of a leaf would reduce the energy available to the plant. Likewise, constantly damp leaves provide a ripe environment for fungus to grow. Many leaves have special air chambers that fill like a balloon and keep the leaves afloat.
Photosynthesis is an essential process for plants and, indeed, all life on Earth, as it makes the sun's energy available as sugars to plants and the organisms that eat them. Photosynthesis requires sunlight, carbon dioxide and water, and while water is never in short supply in an aquatic environment, sunlight and carbon dioxide can prove challenging. Submerged plants often use a special process where the carbon dioxide produced through the plant's respiration is recycled for use in photosynthesis. Submerged plants also draw carbon dioxide directly from the water.
Aquatic plants have developed many reproductive strategies to compensate for the challenges of an aquatic environment. Some submerged plants grow flowers above the water on tall stalks or even engage in underwater pollination. However, most plants use vegetative rather than sexual reproduction, sending off new shoots or breaking into pieces, each of which develops into an independent plant. These mechanisms are so successful that, as pointed out on the Mildred E. Mathias Botanical Garden's website, some species like duckweed can double their population size in a matter of days.