Have you ever wondered if plants that lived underwater still needed sun? Well, seaweed does. Even though it's an ocean plant, it still has photosynthesis. Strangely, though, it doesn't have a vascular system.
In plants, a vascular system transports sap. In humans, blood. Instead, the entire seaweed plant absorbs minerals and nutrients directly from the ocean water. That way, it does not need a system in place to carry sap through its body. Water baby's breath (Alisma subcordatum) enjoys a wet environment. Find it among our wetland plants.
Source of Information on Seaweed