Cut potatoes into ¼–½-inch sticks. Place in a large bowl and cover with cold water. Allow them to soak for at least two hours, up to overnight.
While the potatoes are soaking, make the dip. Combine the sour cream, mayonnaise and buttermilk seasoning and mix well to combine. Set aside in the refrigerator until ready to use.
Drain and arrange the potatoes on two baking sheets lined with paper towels and blot to dry thoroughly.
Heat a few inches of oil in a heavy pot to 300°F. In 3 or 4 batches, cook the potatoes until soft, 4 to 5 minutes per batch. Remove each batch and drain on dry paper towels.
Raise the temperature so the oil reaches 400°F. Fry the potatoes in batches again, until the potatoes are crisp and golden brown. Drain and season with sea salt. Serve with the Ranch Buttermilk dip.