Such plants, like mosses and ferns, are limited to moist environments. As are some animals, like amphibians, which must return to the water for at least part of their life cycle. In a very basic sense, many terrestrial organisms have never actually left the water. They live in the thin films of water that cling to moist places, like the tiny pore spaces between grains of moist soil. Gravity is another basic fact of life that is not a very big deal in the ocean, but of paramount importance on land.
Aquatic organisms rely on the natural buoyancy of water to support their weight.
In general, they donut need to invest much energy in support structures. Unless, of course, they need to move very rapidly, like vertebrates. On land, gravity requires a support system. Plants developed the root-shoot system , roots holding you in place while the stiff tissues of the stem lift your body up into the air. Getting rid of wastes is not a big problem in the ocean - dump it overboard.