There are three types of plants to consider. Submergents These are plants that have their roots and leaves entirely underwater. Submergents add the most oxygen to the water, and require sunlight to penetrate the water. They provide good cover for aquatic organisms. Floating plants have their root systems underwater but their leaves float on the surface.
Water lilies are a good example of floating plants.
Emergents Emergents are rooted under water and have their stems and leaves above the waters surface. Cattails are emergent plants. Emergent plants do not add oxygen to the water. When selecting plants, pick one or two of each group if your pond is large enough. It is a good idea to include submergent plants in any pond because of the amount of oxygen they provide.