Pressure and Continuity
Water pressure increases with depth which varies in the ocean from 1 atmosphere where at the surface to 1000 atmosphere at the greatest depth. Pressure changes in the sea are 'several times greater than that on land and so have a pronounced effect on the distribution of life. Since organisms are limited to surface waters, where pressure is not so great, others are adapted to life at greater depths.
The sea is a continuous body of water. All the oceans: Pacific, Indian, Arctic and Antarctic are connected together. However, temperature, salinity and depth seem to act as barrier to free movement of marine organisms.