Seaweed is a microorganism that lives in the ocean. It is a type of algae that grows in depths up to 40 feet. Seaweed has root-like structures called holdfasts that allow it to hold to the ocean floor. However, holdfasts do not act as roots and do not contribute to the plant's survival in any way. Aquatic animals eat seaweed and humans use it in lotions, soaps and toothpaste.
Kelp is a brown algae that grows near the rocky shores of the ocean. Kelp is able to hold on to the rocks with their root-like structures called holdfasts. Kelp is very fast growing and grows upward toward the surface of the water. Each leaf contains a gas bladder that acts as a plant stabilizer and helps it remain vertical.
Seagrass is a flowering plant that grows in shallow water so that it is able to have access to large quantities of sunlight. Seagrass has roots implanted in the ocean's floor that provide support and nutrition to the plant. Small fish and lobsters live in seagrass to remain safe from predators.