Wind-caused coastal upwelling along the western margins of continents brings nutrients from deep water and results in areas of high productivity. Open ocean waters are deep blue because they contain few particles. Phytoplankton are small, single-celled plants. Individual cells in all organisms regardless of size are small for these reasons. Some others increase buoyancy with a drop of oil.
Viscosity is internal resistance to flow of a liquid.
Maple syrup and honey are viscous liquids. Organisms float more easily in a more viscous liquid. Both increased salinity and decreased temperature increase the viscosity. Flotation is easier in cold waters for these reasons. This is an important component of the much greater stability of the ocean compared to the land.