The Marine Realm includes all connected saline ocean waters characterised by waves, tides and currents.

These processes transport resources and biota over short temporal and local spatial scales (waves) to global circulation over centuries (deep ocean currents). The availability of light and nutrients diminishes along depth gradients. Oxygen may be locally limited in zones of high heterotrophic activity. Salient ambient environmental factors include substrate type (hard vs soft), which influences the traits of benthic fauna and geomorphology, which influences the movement of deep currents and local or regional upwelling. Upwelling is critical to the supply of nutrients to euphotic waters, and hence productivity and trophic complexity. Primary productivity is contributed mainly by planktonic algae, but benthic macrophytes can make important contributions on parts of the marine shelf. Trophic interactions are critical to the structure of many marine ecosystems through top-down regulation.

The marine realm transitions to shoreline and supralittoral, subterranean tidal, brackish aquatic and brackish tidal systems.