To open with my usual One Piece Disclaimer, I am assuming anyone reading this has either read my previous One Piece Reviews as I've been watching the show for the first time or has already seen One Piece. As such, I won't be going into too much detail about the world, and just focusing on this arc. Previous arcs may be spoiled if necessary. That out of the way, on with the review.
So, I am about to praise this arc a fair bit, but I need to get something out of the way right now. Because this arc did something that infuriated me, it starts halfway through a filler episode. I was so confused when I started the arc wondering 'Why are they sailing up a mountain?' After finishing the two episodes I went back to the last filler episode I skipped, and the story seemed to finish up at the beginning. So to understand how people even get into the Grand Line and why it's so difficult, as well as understand what the Redline is, I had to watch half of an episode that starts in a filler arc. Fuck you One Piece.
That out of the way, we finally start to get a picture of what the Grand Line even is, something we honestly haven't gotten that much information on up until now. We know it's a dangerous ocean, but beyond that, it's just been the place where One Piece and a bunch of dangerous pirates are. What we learn now is that the entire ocean is walled off from every other sea. However, each sea connects to the Red Line, a high mountain peak that, at each sea, has a river that flows up towards the top (There is no rational explanation given for this in the arc, and once you reach the top you go directly down into the Grand Line.
Furthermore, it's established currents are virtually unpredictable, and there is a magnetic field that messes with a normal compass, making typical navigation worthless. And while the encounter with the Island Whale with a doctor living inside of it seemingly came out of nowhere, in this case, it works out quite well as it works to establish the nature of what to expect once they enter the Grand Line proper.
We don't really see anything new out of any of the characters, or anything all that special happening during this arc. It was short, sweet, had some fun action beats, and was here for the sake of establishing what the Grand Line was.