There are many false assumptions that Surface people make about the people who spend nearly all of their lives submerged under the water. Most common of these assumptions is that the Sea Peoples do not exist, which is an obvious falsehood on the face of it. Second to that being that life under the sea was much like living on land, which even the most hardened landlubber ought to have suspected was more than a little bit fishy.
In truth life under the sea was anything but like life on the land owing to the fact that water is a wholly different medium that is thick, heavy and viscous and only permits normal visibility to within the first one hundred fathoms of the surface. The farther down you go the darker and murkier the water becomes with visibility limited to barely a few hundred meters in any given direction. The ocean is also teeming with life and has microorganisms growing everywhere that feed the higher life forms with nutrients and predators abound around practically every square mile in a spiraling radius that reaches out from you to incorporate practically everything around you.
The deeper you went the harder it was for normal organisms to cope with the changing pressure, and at one mile in depth the weight of a cubic inch of surface is truly staggering which is why very few of the devices of man could exist at even greater depths.
Normal human lung capacity would be collapsed within the first couple hundred meters, and lungs that required oxygen fed by tanks risked the danger of the bends from rapid changes in elevation with air molecules in the bloodstream threatening to rupture veins like an out-of-control seltzer bottle if traces of such elements as nitrogen could be found at such intense underwater pressure zones. This was why the lungs of Aquanoids automatically collapsed in favor of a more complex oxygen filtering system that copied the ability of fish to extract life-giving quantities of air from tiny bubbles found in ordinary water.
It was also why the Sea People did not put a lot of stock in such vanities as clothing since they were comfortable at home in a watery environment. Muscles tensed and skin grew hard like the overlapping plates of a fish’s scales, only much smaller and more refined. At a thousand feet the weight of the water was equal to a Rolls Royce Luxury Limousine for every square foot of area, which was why the average denizen of the deep was so much more powerful than a Landsman with bodies dense enough to even resist the impact of normal lower-caliber human bullets.
Even softer tissues, such as female breasts, were easily compressed by the equal distributions of pressure so that a Mermaid might go topless without fearing for drag co-efficiency as they skin just naturally softened in areas to create the sort of frictionless surface that allows a dolphin to glide effortlessly through the water. Their lower body flexed quite easily to provide abundant propulsive force while their upper body---human in contour but streamlined like a plane---allowed them to make rapid turns and maneuvers that even dolphins might find difficult to execute which was why they were so elusive. The quite literally could move so fast in water that if a casual diver spotted them they would not have the time to snap their picture.
Most of the Aquatic peoples of the sea had legs and could walk for a brief time on the land since they still needed to perform certain functions that would be all but impossible to execute under the water. Among these skills was the ability to forge metal, which did require air chambers in which to perform metallurgical functions. As all Amphibians are sensitive to dehydration the amount of time that a Smith might work the forge was itself limited, but by working in shifts the craftsmen of the sea were able to create items of rare beauty out of the abundant resources that could be harvested from the ocean bottom.
And to build a civilization under the sea one needs the tools of an industry that is self-sustaining, or else one might as well live like one of the Nomadic peoples who roam the deeps out of sight and mind, only surfacing when some marauding group decided to pay their respects to their more prosperous neighbors who happened to have all of the things that came with a civilized existence.
Generally the people of the Sea had grown accustomed to living beneath the thermal envelope of the two thousand-foot level mark because they had grown accustomed to living at that threshold, and so had moved largely unnoticed by the surface world for the past twelve thousand years. That was why there was scant mention of their existence as they moved about on that part of the ocean where Surface man very seldom traveled, and were it not for the occasional ship sinking the people beneath the sea would have had little knowledge of the world that functioned above their notice.
Of course when something weighing hundreds or even thousands of tons does randomly come raining down upon your heads it does tend to get attention, as had been the case during the Second World War, when surface ships were being sunk in droves, and even one such wreck landing on your house can be enough to provoke outrage in the citizenry of the various habitats and communities so affected.
More recently the chemical and biological pollution of the Landsmen had provoked more than a few of the Underwater Kingdoms to band together in seeking to protect themselves against the Surface dwellers. Simply discovering, through Atomic tests, that the people of the Land had developed the capacity to destroy all life on planet earth had been enough to end their millennia long silence and isolation. And now many Kingdoms had active UN representation where they could vent their grievances in a public forum.
That was another thing that surface breathers just refused to "get" about living underwater. Sound was carried by the medium of the Ocean in ways very different from Air breathing, and most aquatic races were adapted to a form of communication that sounded like clicks and whistles in the ears of Landsmen. One of the initial problems that the Sea Peoples had in communicating with the Landsmen was in overcoming this severe handicap in that each side perceived speech in an entirely different manner. In the Sea you could converse for miles with a sufficiently strong enough sonar projection system whereas radio waves did not travel quite so far without heavy breakdown. Sonic speech was also very fast and efficient in conveying one’s meaning in precise detail, while the polyglot languages of the Surface Dwellers just seemed to guttural and limited, subject to all kinds of ambiguous meanings with miscommunication being more the rule than the exception.