Sun is a limiting factor in water ecosystem because there is less penetration of sunlight in water. Sunlight can only penetrate water up to a smaller depth below the surface. Producers require sunlight to give oxygen and other needed materials that will sustain consumers.
Sunlight, through photosynthesis, provides energy for the most basic of lifeforms and ecosystems that depend on it can only exist down to a depth where the suns rays can reach. Although thermal vents sustain their own ecosystems, independent of the sun.