Huh. No HDL in this episode. Again, I ask: a unique occurrence? You might wonder: why the heck would Scrooge leave the kids at home but bring an incompetent like Doofus? The answer is: so that we can see three characters of dubious competence--Doofus, Launchpad, Gyro--team up to save the day. Which is I guess an okay idea, even if the narrative contrivance is pretty obvious, but the episode does rather excessively play up their uselessness for much of the episode so that their ultimate victory can be more impressive.
The episode has a fair few similarities to Barks' "Sunken City:" trying to recover money from the ocean floor and being captured by fish-people who may or may not be the descendants of Atlanteans (Gyro suggests that the nearby city is Atlantis, but this is never verified). It is indeed a marginal improvement on "A Whale of Bad Time"--our heroes' ultimate escape plan is kind of creative, albeit goofy--but I wouldn't call it great. The fish-guys are pretty charmless, and while the dolphins that Doofus befriends are cute enough, I suppose, they don't really make that much of an impression. At any rate, the kaiju-type monster that the ducks have to vie with is pretty cool.
-Funny opening in which--after another disastrous landing--Scrooge and Launchpad spin into Gyro's house.
-Launchpad, on reading the bottom of the "Marinara Trench:" "Ground floor--pots, pans, ladies' lingerie and lots of mud." An Are You Being Served? reference in a Ducktales episode? Never would've seen that coming.
-"Thanks to Flintheart Glomgold, my fortune is sitting on the bottom of the ocean!''--Well, as noted in the entry for the previous episode, you've gotta take a lot of the blame for that yourself.
-Doofus refers to "my mom," the only piece of evidence so far that would contradict my theory that he's an insane vagrant. Still not entirely convinced, though.
The makeshift scuba helmet that they fit out for Launchpad is obviously not remotely close to watertight.