The North Pacific Ocean pops up in the story because that is where Walton is hoping to sail to through the Arctic. It seemed to be his purpose through the book that nobody has been able to achieve before, to discover the unknown, which is a parallel to Victor's life obsessing over discovery and glory. With the wealthy marine fur trade, the search for a Northwest Passage, and the desire to control strategic territory were among the commercial and imperial goals that characterized British involvement in the North Pacific Ocean in the 1800s. The area was viewed as a crucial location for competition with the US and other European nations, as well as a gateway for trade with China. Through my research it seems like the British really wanted to find a passage through there to have fur trade, so it seems right that in Frankenstein there is a man who is so determined to find it. 

https://www.washington.edu/uwired/outreach/cspn/Website/Classroom%20Materials/Pacific%20Northwest%20History/Lessons/Lesson%203/3.html#:~:text=Consider%20by%20contrast%20the%20British,of%20the%20territory%20more%20aggressively.