These varieties are the most commonly asked about. They will grow permanently under water with the majority of the foliage submersed. Many plants however will have leaves and in some cases flowers that will float on the surface.
A common name used for these plants is 'oxygenators'. This name is somewhat confusing as they do oxygenate the water by day but they also produce carbon dioxide in the water at night. We prefer to call them submerged plants.