What are the only true plants that live completely submerged in the ocean? kelp beds microscopic algae seagrasses mangroves?

seagrasses are only true plants are completely submerged in oceans. they hae various structure that help the survive the environment underwater. they are regarded as true plants because they can reproduce using flowers, they have roots and specialized tissue that help them transport nutrients just like land plants. just like the land plants they need obtain food using the process of photosynthesis and as result they require sufficient amount of light. so as much as they are completely submerged they are often found i shallow parts of the ocean where they can access light easily.