a. Of or pertaining to the deeper parts of the sea; as, a deep-sea line (i. e., a line to take soundings at a great depth); deep-sea lead; deep-sea soundings, explorations, etc. [ 1913 Webster ]
2. at some distance from the shore; as, deep-sea fishing. Syn. -- offshore. [ WordNet 1.5 +PJC ]
3. taking place in the deeper parts of the sea; as, deep-sea exploration. [ WordNet 1.5 ]