I really thought that the implied presence of the monster in the beginning of Upsilon was pretty much perfect. After watching other players go through it, I think that it's also really neat that it can actually show up if you open the door it's pounding against too soon.
The only thing I think could've needed improvement were the smaller underwater enemies with the red light (unfortunately I don't know their name). I had a hard time figuring out their behavior, and I was able to just avoid them most of the time. Maybe you didn't intend to have them be a particular threat, but showing how they act would've been nice.
Then again, you know better than me what you were trying to do. Great job overall!