Well, let's look at the producers first; they're at the base of the food pyramid, so once they're there, a full ecosystem can form.
Plants need four basic things to grow: air, water, nutrients and sunlight. You've got air, you've got sunlight and I'm preeeettty sure you might have some water around there. The problem is nutrients.
Plants evolved on Earth from algae and other simple flora - which are aquatic. They get their nutrients from carbon, oxygen, nitrogen and phosphorus. So, there's oxygen in the water, but the rest are not present. So, you need to obtain the other minerals from something external to the planet.
This could be two things - minerals present in meteorites or artificially introduced minerals. I presume you want the evolution to be all natural, so the idea of a sapient race inserting carbon, nitrogen and phosphorus into the planet's seas may not be satisfactory.
I haven't read the other answers yet, but I wouldn't be surprised if someone else hasn't followed the same intellectual yarn until reaching the conclusion that meteorites are probably the most likely option. The bolides could be laced with nodes of these crucial minerals, which are expulsed into the surrounding ocean, and thus, space-age aliens.
Well, not really. That gives us the building blocks for simple, algal life-forms, but there's still a way to go before fish-people can invade the Earth. We call that way evolution.
So, my original taxonomy of evolution is that it is made up of 3 very closely linked processes - improvement, complication and selection. Note: I am not an evolutionary biologist and do not claim that this is an accurate allocation, it's more of a simplified way of breaking it down.
For plants, improvement usually means more efficient reproductive systems, allowing them to multiply at a higher rate and proliferate. However, plants will also improve their systems for remaining buoyant, consuming nutrients, supporting their structure, transporting nutrients and photosynthesising. You may know that algae are unicellular, so such systems are less tangible, or sometimes non-existent. I recommend you look at how algae evolved into multi-cellular plants to get a better idea of how these systems would come about.
So, the next process in my categorization of evolution is complication. In looking at this picture by the venerable speculative biologist C.M Kosemen (You might know him as Nemo Ramjet.), you can see the vast difference between the first generation of species and the eighth one.
Similarly, if we look to the real world, the anatomy of a Haikuicthyan are vastly simpler than that of a modern Sockeye Salmon. This is very much linked to the last process - improvement - because in improving, the systems of an organism must become more complex. So, to summarize it, over time your plants would evolve into more complex forms.
Now to the third process, selection (again closely connected to the previous two mechanisms.). Natural selection is of course the way nature deals with the incompetent for survival. These include genetic aberrations whose anomalous characteristics hinder their survival ability. For example, there's a reason you don't see many wild albino animals - they simply don't live as long in the wild. However, genetic anomalies can be favoured by selection as well. For example, look at Hemeroplanes triptolemus, the famous snake caterpillar. It mimics a poisonous viper to deter predators. They probably evolved from few caterpillars with an unusual pattern of spots on its head that looked vaguely like a snake. These caterpillars would be eaten less often, and the more spotty caterpillars, the higher chance of them breeding with eachother, and the spots get more and more refined with each generation until you've got a new species.
Then there's another aspect of selection, extinction. In the natural world, success comes in two forms - adaptability and specialization. For example, the simple bivalve genera haven't changed since the time of the dinosaurs. They don't find themselves on the top of the food chain because they're not specialized, but they still survive for dozens of millions of years. Then when you look to the higher layers of the trophic pyramid, you've got the ubiquitous carnivorans, and below them, the ungulates. You could call them successful as well, for they are perfectly adapted for their own environment. But the average carnivore/ungulate genus lasts for only 6-8 million years. They do not deal well with climate change, and the world is an ever-changing place. So, the successful organisms on your water-world will either be simple but adaptable, or advanced but inflexible.
So, you may be wondering, why am I teaching you so much general evolution? Well, Stack Exchange can give you a basic idea of a plan, but refining and continuing your evolution will be up to you. With a synopsis of evolution frequently compared with your planet, this gives you a head-start in the solo territory of worldbuilding.
The basic plants on your world, I think, would look like lilypads, with a buoyant structure to keep them afloat and the other organs submarine. Then you could have plants that grow on their stems, or epiphytes, rather like this real-life tree and the plants it carries:
In one exobiology project of mine, I postulated that floating sponge-like organisms could evolve, and provide a solid surface for "terrestrial" life to evolve on. If something like this might happen on your planet, there could be life like seaweed and coral etc. Also, plants can grow on slow-moving organisms, like the moss on this three-toed sloth:
So, once you've got plants, you can eventually evolve organisms to feed on them. By this, of course, I mean animals. A lot of early animals were benthic, but that's not going to be possible in a seabed-less world. But, all those animals mainly evolved because that's where all the plants were - on the seabed. On your planet, the plants are nearer the surface, so the first animals would probably be simple, buoyant creatures that fed on the plants. The most efficient way of doing that with a worm-like animal (The majority of animals today are worms like Annelids, Platyhelminthes and Nematodes, and they were probably some of the first.) is to have one orifice at the front for consuming food, and another at the back for expelling the waste. Over the course of evolution, worms could turn into bell-shaped animals, giving you cnidarians, or they could become sturdier and form fish-like creatures. The most simple fish today are lampreys, and they do look pretty worm-like, so it's possible that worms could evolve into similar creatures on another world.
Then, you know the drill, the lampreys evolve into cartilage-boned fish and bony fish and all that (I'm not suggesting you rip off Earth's fish, but the basic streamlined design is pretty successful.)
There are some things fish could evolve into that haven't happened on Earth. For example, what if the fish learn to breathe air? And don't think this requires land - just look at lungfish. They evolved "lungs" from aquatic ancestors.
This one was dug up by a guy, alive, encased in a cocoon on dry land.
There are a many possibilities that come with air-breathing. One is that fish could evolve to colonize the topsides of the "lily-pad" plants, to breed or evade predation. This could essentially give you fish that looked like pinnipeds...
Wonderful. The other possibility I thought of is that breathing oxygen supplies more energy to the brain than using gills. This could see a sapient species evolve (I have a personal project centered around a race called the Vendrikk, serpentine aquatic animals that eventually created an industrial civilization. )
To become sapient, they would need intelligence, provided by oxygen, manipulators, which could be claws, tentacles, even prehensile tongues, and collaboration. They could do this to hunt in packs, or for protection.
Well, I've got to go now, and I know it's been a massive answer, sorry if it's too long. I hope it's helped you, and I wish you well on your exobiology project. Happy worldbuilding!