YA(?) story involving a society of brain-modded people and an escapee who returns
I read this story about 30 years ago, when I was maybe 8-14 and it was in my local library. It was likely a young adult story or perhaps adult and fairly short. I do not remember the cover or the names of the protagonist(s).
I'll try to fill in as much information as I can, but things I am particularly uncertain about I'll mark with "(?)".
As far as my sketchy memory goes, the story involved a young man, who was in training to receive a series of brain-mods through computer(s) that were plugged directly into the brain-stem, I think as external boxes. He was training for this for some time, perhaps from early child-hood and there is some apprehension that he may not be able to take the upgrades to a full system. People who have the attached computers do not have much in the way of physical abilities (?).
At some point he learns about people who do not take the upgrades and that they are possibly social rejects (?) or maybe kicked out of the society (?). The young man is able to take the upgrades, but decides to escape the society. 1012
He does this with some difficulty, I think having to swim/float(?) down a river, when he lands he meets some people external to the modded society, some of these people may be escapees too (?) or rejects (?). He stays with them for a while, and gains some physical fitness and skills, but discovers that for some reason both the modded society and the external are failing and the best way he can help both is to return and try to bring down the modded one from the inside, meaning he has to take the mods. He then returns and does take the mods, but I don't remember anything from this point on, or whether that was the end of the story.
I don't remember the end or if this was part 1 of a series (if it was, I didn't read the other(s)).
Any help greatly appreciated.
Edited to add: The accepted answer is definitely the story I remember. It seems I got most of the details correct, but missed the ending - which is the protagonist returning to the society and helping the escapees to get the resources like seeds and tools that they desperately need. It turns out that this was the intended function of the society (called ArcOne), set up to survive an apocalypse, brought on by depletion of the earth's natural resources. The Arc contains all the necessary items to re-cultivate and re-populate the earth. 1012