The episode gives an explanation for the different types of mermaids showing up in previous episodes, as it claims that mermaids become more monstrous and fish-like if they spend more time underwater. Thus, the mer-ducks encountered in "Woo-oo" look the way they do because they spent a lot of time underwater, whereas Launchpad's ex Oceanica probably spent a lot more time on land.
One would think that Webby would be a bit less innately trusting after Lena manipulated her way into their friendship. However, given that Lena was under Magica's thumb the whole time, and sacrificed herself to save Webby, it's easy to see why the lesson didn't land at all.
King Honestus repeatedly crawls on the ground while chasing the ducks, seemingly at times spending as much time on land as he does when on the beach at the end, yet does not revert back to his duck form. Yet, in all those scenes he is technically still "under" the sea, as it's an undersea cavern. One could reason that even if it still counts as being on land, the effect to change him back would take longer to occur.
Della getting bored may be part of the reason that she eventually tried to face her fear and leave the submarine, but it's also very likely that being alone in a metal transport and having to entertain herself in it brought back unwanted (and possibly traumatic) memories of her time on the moon.
Fridge Horror
If Louie's sacrifice tally is accurate, then the kids have each been offered as a sacrifice no fewer than 10 times.
Assuming King Honestus is the cause of those skeletons in the cavern, he now has to live with the knowledge that he killed and likely ate people while trapped in his bestial state.
If Mrs. Beakley has been "fibbing, fibbing, fibbing" to Webby all this time, is she even really her grandmother? On a related note if Beakley's been lying to Webby, who's to say she hasn't been lying to everyone else?
It should be noted that the harp actually delivers the "fibbing, fibbing, fibbing" line after Mrs. Beakley says she won't keep any more secrets from Webby. There are plenty of things a former secret agent would have done that they should absolutely be keeping secret from their preteen granddaughter. There's no reason to think that Mrs. Beakley is an imposter from this, just that she still has secrets she has no intention of revealing.
Mrs. Beakley's line about not keeping secrets comes immediately after her statement that she is Webby's grandmother. The harp may have been reacting to both lines. "The Last Adventure!" reveals that Mrs. Beakley is not Webby's biological grandmother and knows it. Whether she is her grandmother in any other sense is left ambiguous.
The Duck Family decides to leave the Harp, one of the treasures in the journal behind. But, this will not bode well because F.O.W.L. could still get there and steal the harp, and a lie-detecting harp would be a useful asset to F.O.W.L.
"As of Escape From The Impossibin!" this ends up happening.
