The two pieces of evidence that show Harborough considers the shark as belonging in the ocean are:
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“I had invaded a realm which I had no business to invade ... and its guardian was come to punish me.”
This statement suggests that the ocean is the shark's natural habitat and that Harborough recognizes his intrusion into that space. -
“The shark was not passive, in the element, as I was; it was monarch of the waves, it could drive through them with the precision of a torpedo.”
This indicates that the shark is perfectly adapted to and belongs in the ocean, contrasting it with Harborough's own vulnerability and passiveness in that environment.