What do aquatic plants do?

The production of leaves and stems that fuel the food web-they are a valuable food source. The production of oxygen through photosynthesis-they oxygenate the water via plant processes.

Do plants give oxygen to the water? Since they also have one?

Free-floating microscopic plants known as algae, and larger submersed plants like macrophytes release oxygen directly into the water where it is used by animals and other organisms, including the plants themselves.