Fish have highly developed hearing senses. Different species have different means and methods and abilities, so it is hard to generalize other than to say they are good at it.
Of all their senses hearing may be the most important one. Consider that sound travels extraordinarily fast and far in water, as opposed to vision which is limited even in the clearest of waters. Vision is also impacted by obstructions and the amount of available light, but sound is completely indifferent to light and obstructions.
Smell is also important and most fish have well developed senses of smell. But smell is also limited to relatively (to sound) short distances and swirling water currents.
Sound travels faster in water than it does in the air. 4.3 times faster in fact. I've seen this in action. Once I was fishing in the river. About a quarter mile away a man was driving steel tee fence posts into the ground with one of those hammers that's a pipe with two handles and a heavy end. I could see him raise the hammer up and then strike, and then wait for the sound which came considerably later. There were some small bluegills right in front of me and they were reacting to the strike on the tee post. They reacted well before I could hear the sound myself. They were reacting to a sound that traveled through the ground to get to the water before traveling through the water. I could hear the strike but just barely. The fish would "jump" sideways every strike.
Just this weekend I was fishing in the river and I heard paddles hitting kayaks and later two men talking back and forth. I could see nearly a mile up and a mile down stream, and didn't see anyone. Eventually two kayaks came into view. These two guys were talking to each other at a normal volume, and the sound was carrying so far because it was over water and it was a quiet day. I wondered how much of that the fish could hear, and how much it might affect them.
I've seen hundreds of kayaks go by over the years and these two were pretty quiet in comparison to most kayakers. Most kayakers aren't fishing, just cruising.
Fairly often one of the men would make the paddle hits kayak noise. These sounded like bass drum beats, as I guess a plastic kayak actually does become a drum if you think about it. I have no doubt the fish were disturbed by that sound, and that sound would carry even further than the men's voices did. I heard those drum beats long after their voices died out, long after they were around the bend and out of sight.
In the 45 minutes or so I could see them I saw them catch two fish, two small bass. They were both steadily casting the whole time. The covered nearly two miles of river, averaging one fish per mile (since that was for two men that would be an average of one fish every two miles per fisherman) that I could see and I don't think I missed much, if any. Would they have caught more if they'd been real quiet? I think so, I really do. By comparison I caught several while they were in sight, but not when they were close.
So I'm wondering if these relatively quiet kayakers sounded like a bass drum parade to the fish up and down the river. I'm thinking probably yes, yes they did. I figure the fish heard them a couple of miles away, coming and going.
Stealth is a great benefit to a fisherman. Sounding like a bass drum parade isn't stealth.
I've talked before about how fish react to objects above the water, being as "unvisible" as possible is also part of stealth.
A brightly colored kayak moving across the surface with a man sitting up on top of it waving his arm and pounding on a bass drum? Well...
Where I fish the river has primarily a gravel bottom. I can hear the gravel crunch under my feet as I move if I move clumsily. Even when I move with maximum stealth I'm convinced the fish still hear that crunching sound as a distinctive and alarming sound. I'd bet they can hear it a good quarter of a mile away too. There's not much I can do about that except to step as carefully and slowly as I can, and pause when I fish, keeping my feet still. I do that a lot, I'll move soft as I can to the next position then stay there for a long time casting away.
That has the added benefit of slowing me down so that I cover the water much better, and that has been a boon to my fish catching percentage. To put it simply I catch a lot more fish by fishing each piece of water thoroughly and persistently and for quite a good bit of time. Unlike the kayakers that at their speed of travel never cast to the same area twice, in their case it was more like each cast was 50' from the last and next one. But they were having fun in their way as I have fun in my way - so I'm not criticizing, just saying they probably miss a ton of quality fish and pointing out how sound and sight work against them. Those big fish didn't get big by being brave in the face of loud noises and bright lights.
Another time this past weekend I was fishing, alone as far up and down river as I could see and hear, and catching a good number of big bass. Then two ladies with two small daughters came out into the river to float around in blow up tubes. The talked constantly and loudly, the kids screamed and squealed shrilly and loudly and constantly, there was a continual splashing of the water by all four of them. They couldn't have made more noise if they had tried. They were at least half a mile away. I didn't get another bite after they came out to play, and I'm sure it was the noise. So I am convinced that stealth counts, yet again, based on observation.
Those are some of my thoughts. If catching fish is what you are after, silence is golden.