These characteristics suggest a fixed aquatic plant. They have long, hollow, light, and flexible stems due to their nature and in order to survive their environment. Without these features in aquatic plants, it will be very difficult for them to live successfully in water.
Some of these features allow them to float on the surface of the water. They have a hollow, light flexible stem in order to survive the impact of strong currents. With flexible stems, they can easily move with the flow of water. Also, they always have their roots affixed to the ground so that they don't break with strong currents.
Aquatic plants can be found in freshwater as well as saltwater. However, there are some that can only survive in freshwater, while others can only survive in saltwater. With their leaves floating on the surface of the water, they can easily make their food via photosynthesis.