Why are aquatic plants useful?

Aquatic plants provide oxygen, food, and shelter.
The health of submerged aquatic vegetation is an important environmental indicator of overall ocean and estuary health. Seagrasses in bays and lagoons, are necessary to the success of small invertebrates and fish.

If there are no plants, the underwater creatures won’t have oxygen? Is it correct?

Without photosynthesis there would be no supply of oxygen and slowly the oxygen would get used up by oxidation such as rust formation. Without plants, all of the animals that depend on plants would get hungry and gradually die.