Could sapient life evolve on an ocean world? Sure
So long as your ocean is rich in organic life, has heat sources (such as volcanic vents) and mineral access, it's completely believable that sapient life would evolve on an ocean world. Such a creature would develop the ability to move freely in water, breathe and see in water and low-light conditions, and evolve the ability to manipulate its world.
Could sapient life evolve to take advantage of artificial floating islands? Well...
The problem with artificial floating islands is the word artificial. Evolution takes a honking long time — and those islands need to be there and be maintained during all that time. Like, millions of years worth of time.
Worse, sapience comes before artificial, not the other way around. This means if you do have AFIs (Artificial Floating....), someone else not of that world built them and is maintaining them.
But could sapient life evolve on an ocean world and then build AFIs?
Yes, but think about that. We evolved away from oceans onto land. We're no longer engineered to go back. That means we must construct artificial environments (think "domed underwater cities") to live in the water again. Your ocean-evolved creatures, having never had land to evolve onto, would face the same problem. No water to breathe. That makes this explanation kind of a tough nut to swallow, so to speak. What's the point of building a floating fish tank? Now, if there was dry land, building an fish tank on that dry land just as we would a terrarium underwater, that would make sense.
You're developing a fantasy world with magic. You have the privilege of simply declaring the situation to be true. I'd stick with that, because unless you are bringing a third party into the story (the AFI builders) and then allowing magic to change your ocean-evolved dwellers such that they can breathe air (in a way superior to whales and dolphins1), there is no science-worthy explanation for what you want to do.
1 Seals, walruses, etc., don't count in this analysis. Think of them as half-way through the process of evolving away from the ocean. They had land to begin evolving onto. Yes, you could do that — but as I explained, that requires a third party building and maintaining the AFIs for millions of years while your ocean-evolved folks figure out how to breathe air and manipulate rocks and trees long-term.