Assume that we planted floating solar cells on the ocean surfaces. Assume a 100km² solar cell field in the middle of the Pacific Ocean. All of the sunlight will be blocked by them. How would that affect the life in oceans?
The vast majority of light only penetrates the top 200 metres of the ocean, the euphotic zone, so your answer is roughly "what ever lives in the euphotic zone which requires light." In all likelihood I would expect this to cause:
Overall, the impact would be low because the area of the pacific is >165,000,000 km2 so the density of biota would only increase marginally in the top 200 metres of the pacific (the sunlit water volume decreases from 33,040,000,000,000 cubic metres to 33,039,080,000,000 cub. metres). Also, if we assume that the platform is free floating that means it would drift thus only causing short term loss of sunlight for the less mobile organisms.
EDIT: Upon reading @Kotatsu's answer I feel they have made a very good point and I should add the following:
By providing a solid surface it could also benefit oceanic life by increasing area available to live for sessile organisms (just like shipwrecks, oil rigs etc.). Further if the panels were broken up (I assume this would be the likely design because sitting a huge sheet across the ocean will leave it prone to twisting by wave motion) light would be able to pass between the gaps. I agree with @Kotatsu, and this "floating panel" could even be beneficial to the ocean ecosystem.
It might, however, result in an increase of macrofauna, especially if the solar plates were anchored somehow at the bottom of the ocean. This may seem paradoxical, but for most groups of sessile macrofauna (cnidarians, tunicates, bryozoans, brachiopods, some molluscs, and so on), the limiting resource in the ocean is space, and any solid or semisolid surface is a good surface to live on, so at least along the margins of the solar plates, we should expect to see enormous amounts of macrofauna, in the same way as we see lots of it on the keels of ships, on jetties, oilrigs, and all other man-made structures in the sea.
I would expect that once such a sessile community is established, it would be beneficial to other organisms as well. Many polychaetes, molluscs, arthropods, and other groups are predators of the sessile community, and as the larval stages are often planktonic, I would expect to see these established in this community as well, given time.
This could in turn attract fish, especially if the platform is not too far from land, and is not cleaned, but the various shells and structures built by the sessile community is allowed to remain even after the actual organisms die off. The spaces between and inside dead sea shells are often very important places for egg deposition by fish, which may or may not remain in the area when they grow up.
Now, if the solar plates were divided into smaller connected discs, so that there is sunlight reaching the water between the discs, this margin effect would be even more profound, I would expect, as there would be sessile macrofauna around all the margins, and if sunlight comes down between the discs, that would allow some primary production even within the solar field.
So while rg255's answer is certainly also correct, I do not think the situation is necessarily as bleak as depicted in that answer, however, it would all depend on how the structure is built and maintained. Dividing the solar panels into smaller, connected units would be much better than having one solid platform (I assume this would be true also for mechanical and engineering reasons, but I'm not an engineer...), and there are certainly other ways to reduce the impact of such a solar system platform.
I would also guess that any platform placed in the middle of the ocean would have to include some system for keeping oceanic birds away.