The sea represents the sublime, that quality in certain features of the natural world which fills the Romantic soul with a sense of awestruck terror. The sea, like a thunderstorm or a majestic, snow-covered peak, confronts the individual, making him or her feel small and insignificant.
Even a scientific genius like Frankenstein, who believes that he can conquer the world with his race of monsters, can only yield to the power of the sublime as it manifests itself in the icy wastes of the Arctic. In an astonishing act of hubris, Frankenstein tried to control nature, but now, as he recovers aboard Walton's ship, he is as subject to nature's awesome power as much as anyone.
The Arctic Ocean is terra incognita, a part of the world yet to be fully explored and fathomed by man. As such, it symbolizes the inherent mystery of the natural world which will always retain the power to assert itself against man, no matter how much scientific knowledge he gains and no matter how much of the globe he manages to traverse.