1) Below the ocean surface 2) In the depths 3) Like a Cousteau exploration 4) Like much plant life 5) Like some phone cables 6) Like the titanic 7) On the ocean floor say 8) Spear-fishing milieu 9) Subaquatic 10) Submarine 11) Subsurface 1) Coral 2) Submarine 3) Subsurface 
located, carried on, or used under the surface of the sea: undersea life.