Density of water is similar to density of bodies of marine organisms.
Buoyancy of water helps keep floating organisms at surface. However, tissue is slightly more dense than surrounding SW so without special adoptions, organisms would sunk. Snails float by producing gas bubbles in intestine or form bubble rate to float at surface - shell very thin, light and fragile. Algae secrete gas bubbles or have gas filled floats to keep them in sunlit surface waters to maximize photosynthesis. Cephalopods chambered nautilus and cuttle fish regulate buoyancy.