Chop the potatoes into small bite-size pieces. Add to a pot and cover with water. Bring to a boil, then reduce heat to simmer until potatoes are tender when poked with a fork. Drain from liquid and set aside.
Chop the onion, apple, pickles, herring, and beets into bite-size pieces.
Mix together the marinade: add white vinegar, sugar, water, salt and pepper to a bowl.
Add cooked potatoes, chopped herring, pickled beets, apple, pickles, and onions to a large bowl. Pour the marinade over top and toss everything together.
Cover and chill in the fridge for 2 hours.
Mix together sour cream, milk and dill in a small bowl.
Top chilled herring salad with a dollop of dill sour cream. Serve chilled and enjoy!