Deepdwellers are some sort of amphibian. They
The Great Cetaceans live in the same general area as the Deepdwellers, albeit even further down in the depths. The Cetaceans, too, are sentient, with minds so vast and strange that it is almost impossible to communicate with them, but they do try to reach out to other undersea sentients. Not surprisingly, the Deepdwellers regard the Great Cetaceans as gods. A few among the former are chosen and trained for many years to be able to communicate with the latter. When their training is complete, these few are know as Deepspeakers. Others are trained as "Boomroarers", able to belch out subsonic blasts that analyze and detect Sparky devices.
Somehow, Doctor Dimitri Vapnoople caused the Great Cetaceans to attack the British Navy and Londinium, starting what the British refer to as the War of the Dark Trenches. Albia managed to stop the war, but only after killing three of the Great Cetaceans. Diplomatic channels were opened, but the Cetaceans made it plain that, while they would no longer attack, they had no interest in talking to any members of the human race. How directly involved the Deepdwellers were in all of that is currently unclear, but as discussed further below, their attitude towards land-dwellers is mostly shown to be hostile and/or suspicious.
In the Comic
Agatha and Co. (evidently) have their first encounter with a Deepdweller during their brief but eventful stay in the Queen's Society's undersea dome, in the person of Trogulus the exchange student. The visit ends with the group fleeing the battle-wracked structure as its self-destruct sequence is activated. Everybody splits into two parties: Agatha and the larger part of the group skedaddle aboard an escape sub, while Hadrian Rakethorn, Dimo, and Maxim remain behind to herd the Queen's Dome Kitchen Staff (and the Agatha-inspired feast they have prepared) onto a second sub. They succeed in their mission but, because of and delays, they leave the dome at the last possible second, getting caught in the destruction's shock wave. They are from Certain Death when the sub is scooped up by the Great Cetacean Ahnkokanth.
Emerging from the wrecked sub into one of Ahnkoanth's audience chambers, they meet an entourage led by the local Deepspeaker (currently only known by the title His Serenity), who is more interested in beating the nefarious drylander Rakethorn over the head with his staff of office than actually talking. Fortunately, his more tractable assistants (among them Gooblot the Boomroarer) tell Rakethorn that 1. Ahnkokanth is aware that Vapnoople is/was inside the dome, and 2. certain as yet unspecified events have caused the Great Cetacean to feel "unnerved", leading to this meeting. (Rakethorn later comments this is the first time since the war that the Great Cetaceans have made any attempt whatsoever to open a channel of communication to humanity.) The ongoing distrust prompts His Serenity to state he will only officially communicate with a trustworthy individual. Unfortunately, he sees a rather fanciful sculpture of Agatha-as-a-mermaid created by the kitchen staff, and announces that this vision of grace and wisdom is the one to talk to.
But fortunately for all concerned, Agatha's sub is collected as well, and while Hadrian explains the situation to The Lady Heterodyne's close personal associate Honoria Shammington, and Agatha's team of Sparks gets to work on a (relatively) suitable costume, Dimo and the kitchen staff keep the Deepdwellers occupied with the feast. In the course of all this, it's revealed that Deepdwellers have 1. a great deal of trouble telling drylanders apart and 2. issues when it comes to gorging themselves with food. This latter revelation leads Dimo to hastily create the tradition of a round-the-wrecked-sub dance party.
And so. Agatha gets strapped into the arduously, brutally, and exactingly weapons-free tail apparatus, and the official meeting begins on a positive note when she and it get a report of all clear from Gooblot. She then learns first-hand that being a Deepspeaker and having a Cetacean talk through your body puts a definite mental strain on a fellow. She is further told that an important Cetacean and ocean-area have become warped and contaminated by some unknown force, leading the Cetaceans/Deepdwellers to seek help and send along some ambassadors with the humans as they return to Albia.
List of Deepdwellers
Possibly Relevant Outside Information
Since the 1950s, Brine Shrimp have been sold as pets, under the name "Sea-Monkeys". The advertisements feature cartoons, that resemble Deepdwellers rather strongly. This may have been an inspiration to the Foglios.