The obvious hitch with a superdense atmosphere is that, even without being super deep as well, it absorbs light. Just as the bottom of the ocean is dark even just miles deep, an atmosphere as dense as water(even if actually in the gas phase) would block out light from the sun.
How might we get around this? I already know about sonar, but besides that I’d entertain alternate modalities that end up with a visual-like sense.
Fish have your answer: the lateral line. Its a sensory organ that runs the length of the fish's body. The lateral line allows fish to sense movement and vibration in water. If you adapt this a small amount you could easily allow for it to work in very thick gasses. The cells on the lateral line have sometimes mutated to allow for electroreception, which is something you should really look into. Electroreception allows for electrocommunication and electrolocation. Which seem to be what you're looking for.
Starting with what constitutes vision. The vertebrate eye consists of an array of photoreceptors with high resolution due to the large numbers of receptors. Those receptors feed their signals into a specialised region of the brain for processing the signals into that pleasing illusion we call sight or vision. It's easy to fall for the illusion and believe we perceive the world directly, because we don't. The vision of the world around us is effectively a natural form of virtual reality. One that we are well truly embedded in, but fortunately for us the environment where we experience vision is the same environment which helped generate it.
The high resolution of the receptors generates the information makes the world seem seamless and lacking in granularity. Therefore, if an alternative form of perception constitutes eyesight it must have the same characteristics. Namely, high sensory resolution and a large neurological capacity devoted to processing the massive signal flow.
Let's start with eyes. There may be some light in the depths of a superdense atmosphere. Perhaps not enough to read by or hunt prey, but if there's just enough light, then it won't hurt for our aliens to have, at least, one pair of eyes.
Infrared radiation is universal. Everything radiates heat. It just depends on how much heat. Snakes have pit-like heat receptors on their cheeks to help target their prey. This is apart from their eyes.
Let's assume an organism with a large array of thermal receptor pits. This information could be processed by its thermovisual cortex into generating a thermal vision of its surroundings. It's likely this thermal vision will be more coarse gained than our visual world, due to the larger size of the heat receptor pits. Of course, if evolution manages to develop smaller and more efficient heat pits, then this will lead to better thermal vision with improved resolution.
Because a superdense atmosphere will be so good at propagating sound, it would be remiss not to consider the evolution of acoustic vision.
Again this would consist of an array of sonoreceptors that would also have the high density of signals processed in a specialised sensory cortex of the brain. While ears have their own arrays of sound receptors an organism with acoustic vision would need to possess many more sonoreceptors. Perhaps its head could be ringed with ears. The sonoreceptors might be more specialised in their frequency capture. Humans and most vertebrates on Earth are binaural in having two ears. Creatures dwelling in superdense atmospheres could have polyaural hearing or multiple ears, which would be better at locating the source and direction of sounds.
It is quite likely that the creature wouldn't just use ambient sounds and noise for its acoustic vision perception. If this was combined with echolocation and sonar, this would greatly enhance the creature's capacity to perceive its surroundings acoustically.
There is no reason why creatures couldn't combine all three of proposed 'visual' systems. Conventional vertebrate eyes adapted to low light conditions, thermal sensory arrays feeding the thermovisual cortex and an acoustic sensory array(s) feeding its sonovisual cortex.
While a creature won't precisely perceive its environment in exactly the same way we do when it comes to eyesight, functionally it should be able to navigate its way through the world because these potential sensory mechanisms constitute sight in all but name -- apart from the fact that only one of them involves light directly.
Added to Edit:
While looking for something else, this article by Robert Freitas called "Extraterrestrial Zoology", originally published in Analog, July 1981, turned up.
Vision, of course, is simply the detection of one narrow set of wavelengths of light within the entire electromagnetic spectrum. One alternative to “visual” sight is infrared (IR) vision, or seeing with heat waves. The rattlesnake is quite good at this – the creature has two imaging eyeballs operating in the visible, and two conical pits on either side of the head which permit binocular IR sensing of temperature differences as little as 0.002 °C. The theory of optics predicts that alien infrared eyeballs with resolution close to that of the human eye could have apertures as small as 4 centimeters at 93,000 Angstroms (the peak wavelength of black body radiation emitted by a warm human body). This compares well with the size of the eye of the Indian elephant (4.1 cm), the horse (5 cm), the blue whale (14.5 cm), and the largest cephalopods (up to 37 cm).
The fact that rattlesnake have binocular IR sensing and that optics can allow for IR eyes having the same resolution as the human eye, then alien organisms living under a superdense atmosphere would be able to see quite well if they evolved infrared eyesight.
Originally the concept that the heat sensing pits on a rattlesnake could be readily scaled up to that of human vision wasn't obvious. However, it did seem likely that hypothetical but plausible way that both infrared and acoustic vision could be possible, provided certain conditions were meet. Now alien infrared vision seems very likely. The aliens might look gruesome with holes in their heads where their eyes should be.
In planet Earth, many deep-sea fish, like the lantern fish, have some sort of bioluminiscence as well as big eyes adapted to the dark. This would work also in a superdense atmosphere.
Obviously, deep-sea animals evolved from others that were adapted to sunlight. So the real question is: why an animal should evolve to see in a superdense atmosphere? The simplest solution is that the atmosphere wasn't always as dense as today. Maybe a long period of intense volcanic activity caused the emission of gases that block sunlight.
