Plant life first evolved in the ocean, and then spread to the land. Many of the things we take for granted in land plants don't apply to plants in the ocean, and ocean plants have several adaptations to help them survive in that environment.
Most ocean going plants have adapted to their environment by developing gas sacs and air sacks to lift their photosynthetic surfaces towards the surface of the water, to collect sunlight. Particularly in the deep ocean, these sorts of adaptations are critical. Examples of this type of adaptation include Sargasso sea weed, which are sometimes called 'sea grapes' because of the visibility of the gas sacs.
Structural Differences From Land-Based Plants
Two major differences between oceanic plants and land based ones are structural. Land-based plants use roots to pull up nutrients and water from the soil using vascular action, and the entire structure of the plant is built around this function. This explains why plants have stems, for example. On oceanic plants, the root structure is called a keepfast, and is an adaptation meant to anchor the plants against the currents, and there is no need for internal water transportation within the plant. Indeed, most oceanic plants are more properly typed as macroscopic algae than vascular plants.
Ocean plants have to be able to handle the dissolved salt in the water; the adaptation mechanisms range from living in (comparatively) low salinity regions of the ocean to complex mechanisms that break down salt slowly into chlorine and sodium ions, or storing it and passing it along with the respiratory products of the plants. This can cause beds of oceanic plants that directly regulate the salinity of the water in their region for their optimum environment.