Could a species of planktonic beings, with sufficient intelligence, creativity, and physical ability to manipulate objects, create a technological civilisation at the same level as humans? While being unable to control their motion is a massive detriment, it seems plausible that they might find a way to work with or even overcome the currents through technology. Please ignore any issues that apply to aquatic beings as a whole
Could a species of planktonic beings, with sufficient intelligence and creativity... While being unable to control their motion is a massive detriment
No, no, no, no, no. Nope. How did they develop "intelligence and creativity" if they cannot even control their motion?
While it is true that form follows function, the lack of a form will disable any potential function.
Case at point - humans inherited opposable thumb from arboreal primates. It enabled them lately to used tools and thus contributed to the development of intelligence. Intelligence needed the frequent use of tools but didn't "create" opposable thumbs as a way to emerge.
Increased intelligence isn't something that magically appears in a species; it has to be selected for... and physically possible.
First of all, a sapient brain is necessarily bigger than a plankton's body. It takes a minimum amount of physical hardware to compute anything.
Second, if plankton can't even control their movement, they don't benefit from increased intelligence, so there's no pressure to increase intelligence. In fact, for a planktonic being, increased intelligence would be a detriment, since it requires a higher caloric intake and (in this case) provides no benefit whatsoever.
If they can link together to accomplish complex tasks then maybe.
Taking ants for reference they can't accomplish much on their own but are an unstoppable force in great numbers. They don't have a civilisation per se but they can build large nests, wage wars between colonies, farm mushrooms and herd aphids. They do this thanks to two things: mandibles to manipulate their environment and pheromones to communicate among themselves. We for example have hands and can talk. Grab and communicate, it's a useful rule of thought when designing a civilisation capable species.
If the zooplankton could link up with each other they could form large schools(?) and grab and manipulate objects. The way they communicate could be anything. Light, scent or sound.
However intelligence is the limiting factor. Multiple tiny brains working together aren't as good as one large brain. Ants can do intelligent decisions but that's because their pheromones are a type of external memory. Each ant only follows basic rules. That's why we still dominate (for now).
They're smarter than you think...
Single dinoflagellate cells (plankton) actually produce an eye with a lens and retina. Having a retina is a good hint that they may keep mental track of the positions of objects, though I haven't seen much work on this.
Now you may wonder what a single cell can do without a brain. But remember, cells have many thousands of different regulatory genes, each of which produces a protein can affect others in a complicated network - a neural network, you might say. With a sufficiently complex protein network, you could have quite a bit of processing power.
What about fire? Well, who needs it? The point of fire is to transform substances - ore to metal, dead chicken to barbecue. If the cells actually did have a sufficiently complex neural network, they could make new protein sequence on the fly to provide surfaces with any desired pattern of catalytic activity - accomplishing all the purposes of fire, and much more, without ever needing such absurdly high activation energies.
Now does this seem far fetched? Of course ... we're writing sci-fi here. You would face equal skepticism if you interviewed a corporation in the future and tried to convince it that the ordinary worker unit, that it had just printed in a sessile cube chassis with specialized software preinstalled and slotted into a rack in a server complex, was actually the descendant of a free-living and "sentient" species that had existed just a few hundred years previously.