Consider that the Sea Folk, the Atha’an Miere, and their lovely little island of Tremalking, are harboring (no pun intended) a deep dark secret. The Sea Folk seem almost to be phobic of human companionship, or companionship of any kind for that matter. They are a closemouthed group of people, and are always alone and separate, they guard their privacy with strict determination. They stay at sea or on their boats more often than not, their children are born at sea, away from prying eyes. They are almost as conscientious of their privacy as the lands beyond the Aiel waste.
Next, remember of their prophecy, no not the fabled Coramoor, but their "Age of Illusions" or rather the "End to the Age of Illusions." Now this is a very curious thing. Take their secrecy as a race, their dark skinned appearence and their closemouthed society and now this ending they speak of on the island of Tremalking after Rand and Nyneave's little Stint with the Sa'Angreal and the glowfest from the half buried statue. I believe that what this means is that the Sea Folk are not human. I believe that they are some sort of creatures (more on this in a moment) and that they have been hiding behind a weave of illusions, making the world see them as human. And now with the Coramoor's arrival and the Sa'Angreal's use, the illusion they have been hiding behind, they are now free of and can once again emerge into the world in their true forms.
Perhaps they have hidden themselves away due to the fact that they fear persecution. How many people ran screaming from Lolial because they thought him to be a Trolloc? Perhaps the fear of racial discrimination had driven them during the breaking to hide themselves away and take on human form to avoid notice. After all if they were to be some type of hideous creature, they would fear that human kind may try to erradicate them, in typical genocidal fashion.
As to who they are, or rather what they are, I believe that maybe they are some type of shadowspawn that turned away from the Dark One and found salvation in the Light. Maybe even Trollocs or Fades, or some other type of creature whose description or race had been lost along with everything else that was lost or forgotten during the Breaking, some 3000 years prior.
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Roonas says: Hmm ... I think someone has been watching a little too much X-Files!
And in a factual question - Are not the Ath'an Miere in a symbiotic relationship of sorts with the people that inhabit the islands they
"protect and own"? If I remember correctly, the Ath'an Miere stick to the sea and shores, leaving the land to be inhabited by a secret society whom are somewhat xenophobic. They make goods and things to trade, the Ath'an Miere merely trade the things for them and thus it is a symbiotic relationship. They are not "one people", even though they might be off-shoots of the same society at one point in the past. I do however think there is a deep dark secret hidden there, but I personally do not think it fits under the "aliens from another planet" idea you seem to be stating here.
If it is true though, then that must be RJ's new job after WoT is over - X-Files Script writer! 
Comments
I don't have any proof to back this up, but I have a hunch that the sea folk were once Aiel.
I believe that you may be thinking of the Amayar. Thos are the people that live on the island of Tremalking (and possibly other sea folk controled islands). Their beliefs in a pacifistic philosphy called the Water Way is very similar to the Way of the Leaf that the original Dashain Aiel lived by. Also their view of the physical world as a mere illusion is very close to the Aiel belief that life truly is but a dream.
By the way one hole I can punch in this theory would be that we know of one Sea Folk gal (Shalon somethin or other) that has had carnal relations with one of the shore bound as they refer to all others. If the sea folk were different creatures with illusory weaves then one would think that Ailil (the Cairhienin woman that Shalon had an affair with) might have noticed something odd and told Verin about it.
If you're talking about the Amayar living under illusion weaves then that is a little more plausible, but only a little. I will also agree that whatever secret they hold, they are still human.
Sooner or later it all connects back to itself.
First of all you should not mix the Sea Folk and the Amayar. They are as different as anyone could be, both in looks and religion. You say they are dark, yes the sea folk are dark, but I think I can remember the Amayar being fair. Also, you talk about the time of Illusion as a Sea Folk prophecy, but I seem to remember it being an Amayar one.
I agree that the Amayar seem like the Aiel that fled in boats after the breaking, or perhaps the Sea Folk. I'm not quite sure about that..
If, however, I should accept your theory as being the Truth, I would mention Ursule le Guin or something. She wrote some books
about a place called Earthsea, and there you have something called human dragons. That is dragons in human form, but with the knowledge and power of a dragon.
I do not thing that has anything to do with anything, though.
Just rambling on.
Magnus Alexander corpore parvus erat
Dissenting voice of wotmania
Frightfully stubborn pacifist
Violence is the last refuge of the incompetent
Lets sit back and actually think about the odds that they would have been able to hide somthing so enormous. First lets think about the forsaken wouldnt they have possibly mentioned somthing about the fact the fact there are aliens among us
= second how would they have been able to hide their selves from two thousand years of aes sedai contact however brief. but nice imagination.
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An Opptimist Knows The World Is Good... A Pessimist Fears This Is True.
IM IRISH!!
It was the members on the island that had the proficy, not the sea folk. everything i have seen says that the sea folk
will have nothing to do with the islanders. they will trade for them and rule over them, however, the islanders rule
themselves. Doesn't this sound a little like the Jenn Aiel? The Jenn withdrew from the fighting Aiel and made their own little
community. Also the islander follow the water way. Like someone said above they sound like Aile. In the last age, Aiel served
Aes Sedia (sp?). While it has been thought that all of the remaning Aiel left on the carivan route with the an'greal etc., maybe some
were trapped on ships. I seem to remember that a Sea Folk had said that they knew nothing of the water in the begining, but were
blown away by the wind. Wow this got bigger than i thought it would.
CAI
foster brother to Arther Pendragon
Knight of the Round Table
This can not be so. Only humans it seems can be able to harness and use the one power. If they are not human how can the wave mistresses embrace saidar? We know that they had to have weaved flows into the weather bowl to restore the weather so are you saying that not only humans can embrace the source. If so where do you draw the line at Ogier's?
Another example of taking any two random things and making a new theory.
"You seem a decent fellow... I'd hate to kill you."
well it would be cool if this were true...although kinda disappointing, a little too tabloid-ish for RJ. i think its much more likely that they used to be Aiel, and their part in this is that they have "the song". Which is most likely not a real song, its a euphemism like the Aiel calling battle "the dance"
first shadowspawn don't or can't channel and second
what are you smokin?
"Death is lighter than a feather, Duty heavier than a mountain."
1. The Seafolk are secretive to hide their channelers.
2. That a seafolk quoted something an Aiel said does not prove anything. If an Amayar said it, it might be further evidence that the Amayar are from the same root as the Aiel.
3. In Rhuidean, it was a dreamer that said the phrase. Maybe there are dreamers among the Amayar as well: dream-illusion?
The only way to convince me is by providing some evidence!
Beware, Lawbreakers! 
'Tis I, the Thieftaker! 
Faithfull servant of... 
Oops, forgot... I don't serve anyone!
Sea Folk and Aiel related?