Aquatic plants get water and carbon dioxide from their aquatic environment. Even though the plant is underwater, it still gets its energy from the sun because sunlight can pass through water.
That is how strong the sun is. Even underwater, deep down the ocean, plants still absorb it. I wonder how deep it can reach the plants?
Light in the ocean decreases with depth, with minimal light penetrating between 200-1,000 meters (656-3,280 feet) and depths below 1,000 meters receiving no light from the surface. Thus, there are no plants below 1,000 meters.