What is an aquatic plant?

An aquatic plant is a plant that lives in freshwater. Some can live submerged in the water or live floating on the water. Other live on the floor of the pond or lake or wherever it is. They can also live in between drifting. Examples would be lily pads are floating however, aquatic ferns live at the floor. They can live in salt water like the ocean or regular water like a pond or lake.

'A plant that grows partly or wholly in freshwater whether rooted in the mud, as a lotus, or floating without anchorage, as the water hyacinth.' From Answers.com

A marine plant is a plant that lives in seawater.