The plants found in the sea are predominantly algae. The green, brown and red algae, are so called because of the different pigments they use for photosynthesis (and thus the wavelength range of the sunlight they use) are adapted to live at different depths with different light climate. The red algae which use the green light, can survive at the greatest depth.
The so-called benthic macroalgae, as opposed to the free-floating (planctonic) ones, are fixed to rocky bottoms at shallow depths all around our coasts. They need the same nutrients as higher, terrestrial plants and have efficient mechanisms for taking up needed nutrients directly from the surrounding water and thus do not need any proper roots. In areas where the water is contaminated certain species may occur in mass production and contaminants can get accumulated into the plants.