If a marine plant grows deep underwater, how could it evolve to still obtain sunlight? what would the structure of the plant have to be in order for it to thrive?

Its  chlorophyll pigment would be adapted to utilizing the blude-green spectrum of sunlight for photosynthesis rather than the red spectrum like land plants. This is because red light is absorbed by the oceans and does not reach greater depths (this is why oceans appear blue). These plants would therefore appear red in color.

Plants that live in the oceans have air-spaces in their leaves that that help them stand upright in water. They use buoyancy for structural integrity. This is why even long plants such as kelp seem to be upright in water (but will collapse when put on land).