I think if I had to list my top 5 worst dp episodes, this one would probably be on that list. And that's not because the episode itself is that bad, b/c it really isn't. But, anyone watching at the time this episode actually came out knows how disappointing this episode was. It was very falsely advertised as Cressalia vs Darkrai; the preview contained the only fight scene b/w the two pokemon, as if to say, this is only a taste of what's to come. There are some really funny reviews in this thread about the absurdity of this.
This episode marks a transition in the story from Ash back to Dawn. Ever since Fantina's first appearance (which feels like a good few episodes ago), the story has really focused on Ash. A couple of episodes somewhere in that stretch had to do with Dawn, and they were both similar in that they showed Dawn not being able to take care of her pokemon properly, and feeling depressed as a result (one with Piplup and one with Pachirisu). But neither of those episodes are really that fun to watch for that plot point alone, and I think the story is beyond the point now where Dawn struggling with confidence is something that's entertaining to watch. It seemed to me with those episodes that the writers were really struggling to find what to do with Dawn's character. And this is their solution: have Swinub evolve. And it works well (I was really surprised by how entertained I was when I watched some of these future episodes; I knew what they would be about, and I thought it wouldn't be anything I hadn't seen before, but that wasn't the case), especially since it shows a progression of Dawn's character. I think it correlates to the way Piplup has been written in the past few episodes, where its vanity has gotten it into embarrassing, funny situations. And even though it's aware (and Dawn is aware) that this often happens, it still does it anyway. It doesn't mind the ensuing embarrassment, and Dawn doesn't really mind it, either. Dawn is that type of a quirky character; I really think May was sort of that way, too, and this type of storyline might have worked for her as well. I think I tend to appreciate this sort of stuff more from Dawn than I did for May b/c Dawn just strikes me as a quieter, more introverted character. She doesn't seem to me as being someone who's that concerned with how others view her (she cares about what Zoey says, she's supportive of Ash, and she normally says a witty comment to TR, or to Brock-that's really about all she does), and I think that goes to her talent level so far as a coordinator. Once Sinnoh is all said and done, I think it's fair to still to say that the Wallace Cup victory might be the achievement she's most remembered for. Unlike May, Dawn never feels like that superior type of coordinator-why should anyone really have something to say to her, in the first place? When the anime reached the Grand Festival part a few months ago, I think no one really had a clear idea of where Dawn would place, and whether that would be a disappointment or success. For Ash, we knew he would go far, b/c he's shown to be a better/more experienced trainer than just about anyone who's come his way, excluding Paul. How far Dawn would go was much more in the air.
That was quite a tangent. For the episode itself, this story could fit really, really well in the LoZ, the Wind Waker game. Sailing at night has quite the adventurous feel to it, there's islands everywhere, and even a shrine where a beautifully shining pokemon (a sage or something might be more befitting a LoZ game) is be summoned. Of course, there's an evil monster that's imprisoning the sage, so that has to be defeated (TR in this episode). And once the sage is freed, it's power enables the really bad guy to be sent back to another dimension (and since the bad guy is not destroyed, there's room for a sequel). Yep, I could really see this episode being incorporated into a LoZ game, particularly the Wind Waker. That's a good thing, and there's really nothing that wrong with this episode. There's even some fun normal scenes, like Dawn saying Ash should go shopping to get over his repeated rejection of not being able to find the gym leader (and also, I like how the writers are letting Ash's repeated sayings that he's going to win the gym go, and not inserting some sort of moral that Ash is arrogant-he isn't, and he has the right to be this confident; this is his fourth region of gyms, and he's been offered to be a frontier brain-no way he shouldn't be confident, especially after his last victory), as well as Brock skipping to Jenny to find out what's going on in the town. This episode's main negative was the terrible way it was advertised. It was like being told you would get the feather from a Cressalia, and instead received the feather from a Pidgey.