This episode managed to scare me a bit when Hiccup was close to drowning, which isn't typical with Race to the Edge since I'm never worried about any of the characters (or at least the main ones) dying. During that scene though, I was on the edge of my seat. They really managed to create tension there despite viewers knowing that he had to survive.
Toothless swimming down and looking through the glass, refusing to swim up because he wanted to stay with Hiccup, was the most heart-breaking thing. I don't know that I've properly appreciated Toothless so far in my reviews for this season, but he is the absolute best. Toothless is the primary reason I feel in love with How to Train Your Dragon when it first came out. Moments like this remind me why that was.
The Submaripper is also such an intense dragon that you wonder how they'll come up with something to top it in the season finale.