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Adding to Dale Emery's answer, and maybe clarifying it a bit, I would say that the tense depends on the narrator. If the story is narrated by an omniscient narrator, then the ocean is vast, becaus...
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Source: https://writers.stackexchange.com/a/16214 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
Adding to Dale Emery's answer, and maybe clarifying it a bit, I would say that **the tense depends on the narrator**. If the story is narrated by an omniscient narrator, then the ocean _is_ vast, because that is what the timeless and universal narrator knows about it. If the story is told from the perspective of the protagonist, as he experiences the events, then the ocean _was_ vast, because the protagonist perceives it as vast. I feel that most writers do not have a clear understanding of their own narrator and switch back and forth between omniscient and personal narrator, sometimes even between first and third person (like Stephen King in _Carrie_). This works for some books and writers (as the success of Stephen King shows), but depending on your writing style you might want to think about who is telling your story and stick to that viewpoint and only deviate from it if you know why you do it.