This one actually reminds me of my favorite level in the Lego Movie Videogame. That level was also restricted to 2D movement while swimming underwater. Just one level left to go and this game's story mode will be completed.
By the way, after you've completed the story, perhaps you might consider recording one more episode to show some additional content in the game. If I remember correctly, you once already showed us where you can replay levels you've already completed. Some additional things I've seen in that main hub: the upstairs room is where you can rewatch cutscenes, the door to the right I'm sure you know is where you continued the story and I assume you can later simply explore Hogwarts, and the door to the left leads out to Diagon Ally. Along Diagon Ally, you'll find places to enter where you can: purchase unlocked characters and build custom characters; purchase red box abilities you've unlocked (when you find a red box while exploring Hogwarts, just carry it to the nearby owl perch and it'll become available to purchase); enter cheat codes; and play bonus levels and even build your own levels. Perhaps you could at least show us the four tutorials (unlocked after you've completed any of the ten bonus levels and available as an alternative to playing said level again) of the level builder, as I think it would be interesting for all of us to see what was apparently a rather short-lived feature among Lego games (the only other one I'm aware of that has it is Indiana Jones 2).
As for my own progress through this game, I have not yet gotten this far, but the next level I'll be playing will be the final one of Year 3, which you may recall is the one that you had to keep restarting many times due to the game crashing at the same place every time. Well, it seems that so many people have had that issue that someone even wrote a guide on Steam on how to avoid it (the guy claims that watching the ice melt from that door is what causes the game to crash). I really hope that I don't encounter much trouble there, but considering how many tries you said it took you to finally get passed it, let's just say that I'm glad I'll have the entire afternoon set aside.
Formerly known as Takanuva's Symbol, I rejoined BZPower on October 10, 2012.
These days, I am perhaps best known for my obsession with all Lego video games.