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As for appendages, this is where I think you are wrong. Depending on the environment in which they developed, more than 2 arms could be an advantage. Did they climb or move around in an arboreal setting? Having arms to swing from while still having arms to manipulate their environs would be advantageous.
If these fish have a different number of fins, the story could have developed differently.
What sparked the growth of humans was the invention of how to create fire. Which allowed everything from warmth to weapons. Not possible under water.
In the water and other beings, the development of the mind could go another way.
If you believe God is an alien, very likely as we were supposedly created in His image
The new question will simply sound like "Could other aliens look like our alien-god?" If they don't look like Michelle Obama Democrats will proclaim them 'racists' and attack them. No point in them having anything to do with Earth.
I think the aliens will be non-biological since things like DNA seem too fragile to travel in space.
On this, even if a very narrow basis, you can build completely different designs.
Absolutely! But not the smelting of metals and formation into shapes necessary to build tools and spaceships, kind of requires fire.What sparked the growth of humans was the invention of how to create fire. Which allowed everything from warmth to weapons. Not possible under water.
Fire is important for our civilization. And he greatly helped people to protect in ancient times. But I do not think that fire is an obligatory factor for the formation of the mind
Absolutely! But not the smelting of metals and formation into shapes necessary to build spaceships, kind of requires fire.
And the water of fire will not be a hindrance. Earth's water does not interfere with underwater welding. As soon as civilization opens the transformation of mechanical energy into heat or electricity, it will begin experiments with increasing temperature. He learns about boiling water, about gases, about burning and melting. Even if such a civilization is difficult to smelt underwater, it will master the smelting above the surface of the water In the end, it is much easier for a sensible to leave the water element than to leave the atmosphere. We need to build rockets for this, and they just need to stick their heads above the surface of the water 
Maybe we're the aliens.
The first way to smelt metals from their ore requires a very hot heat source. It may be able to be done under water using ocean vents, but first the container must be invented, using carved stone to create the template. And since water is going to absorb the heat from the source a lot faster than air, it will be boiling hot water around the metal that you're trying to form. You need a robot to handle it, and hence the robot must be built beforehand. And then once removed, the heat will escape so fast into the water that it will solidify and cannot be forged to the shape you want, in time.Absolutely! But not the smelting of metals and formation into shapes necessary to build spaceships, kind of requires fire.
If the civilization evolved to the idea of building spacecraft, then the processing of metals this civilization will open and under water
So in order to smelt and forge metal under water into specific shapes to make spaceships, means they would need advanced science prior to the attempt. Which was probably developed above water.
I believe that the first "atmosphere climbers" should appear in such a civilization not much later than the divers have appeared
If you believe God is an alien, very likely as we were supposedly created in His image
This will not be an answer to the question, but a postponement of the question
There's nothing 'special' about our particular shape. There's plenty of 'intelligence' to be seen in dogs, dolphins and birds, and any one of these could, given the right circumstances, and a few million years, be as capable as we are, of raping the planet.
We just got there first.
So the 'aliens' are as likely to look like an octopus, but probably something we would struggle to believe.
But how could they thought of fire when first climbing into the atmosphere?
I, too, once in the teenage period thought that, of course, the universe is so diverse! And there must be some other form, with sulfur instead of oxygen, with silicon instead of carbon. Or life forms generally based not on chemical processes - live computers, etc.
While what you say is probably true, the enormous gravity of a planet like Jupiter generates tidal heat in it's moons. There are likely undersea vents of super-heated liquid somewhere out there. I recall a probe recently pass a Jovian moon and detected H2O being ejected into space.By the way, one more my disappointment after studying of physical chemistry is a hypothesis about, let and organic, but non-aqueous life. Methane or ammonia as solvents. Satellites of the outer planets of the solar system ... The hypothesis is beautiful. But there is such a fact - the rate of chemical reactions depends exponentially on temperature. And the rate of biological evolution depends on the speed of chemical reactions. With a temperature drop of 100 degrees, the reaction rate decreases thousands to millions of times. The planets of the solar system are about 4.5 billion years old. So, according to the "evolutionary clock", stalled 1000-1000000 times, on the cold outer planets has now passed since the emergence from several thousand to millions of years. This is very small for the emergence of even primitive life.
My thoughts are that the conditions that led to human intelligence are so rarely met throughout the universe that we may never met another like us,Absolutely true. There would actually be infinite aliens that look exactly like us and all asking the same question. That's probably why my internet is so slow....If you believe in an infinite universe then there is an alien out there somewhere that looks exactly like you and asking the very same question on an internet forum.I have been arguing for dozens of years that an alien that is capable of building a spaceship, would probably look like us. Not exactly, but they will have similar traits that we recognize as "humanoid". This is due to natural selection and the capabilities needed to build a spaceship. A "blob" is not going to land on earth and roll out of its spaceship.
It has to have the capabilities to build the spaceship, and being that natural selection will favor the maximum and minimum requirements for those capabilities, and the necessities they need to survive and evolve, aliens that land on earth could very well look similar to us. I've been saying this for over 30 years.
So Oxford finally did some science on the subject, and came up with the same conclusion that advanced life on other planets could be similar to us. Of course, it's not proof, but it's nice to see one of my 2170 theories get some recognition. ;-)
Aliens may be more like us than we think | University of Oxford
While what you say is probably true, the enormous gravity of a planet like Jupiter generates tidal heat in it's moons. There are likely undersea vents of super-heated liquid somewhere out there. I recall a probe recently pass a Jovian moon and detected H2O being ejected into space.
Long-wavelength electromagnetic radiation does not affect chemical processes. Short-wave radiation destroys molecules. There is a very narrow range in which the energy is already enough to interact with molecules, but not enough to destroy them. In the process of evolution, it so happened that our vision filled almost all of this range Infrared radiation is already too weak to directly cause photochemical processes. Far ultraviolet is already destroying the organic matter. So we did not get our eyesight unless a small piece of near UV. So the range of visible light has a completely physicochemical meaning 
