deep-sea

Short definition from Wiktionary

Adjective

1. 1
   Of, related to or located in the deeper part of the sea.
2. 2
   Of or related to open sea.
3. 3
   Able to accommodate the largest ships.

Noun

1. 1
   The deeper part of the sea or ocean in which no light penetrates
2. 2
   open sea (part of the sea out of sight of land)