The ocean of my very earth-like planet is covered in its entirety by vegetation, forming a kind of carpet on top of the water's surface. This free-floating halophyte is a superorganism formed by trillions of individual plants. The squishy upper layer always stays on top of the water thanks to pneumatocysts underneath, which help it maximise its exposure to sunlight. Nutrients are absorbed through a forest of low hanging roots which reach just a couple meters under the surface. The plants reproduce not by pollination but by cloning themselves repeatedly.
In the case of ponds, floating plants are problematic and frequently wipe out local populations. They starve the plants underneath from sunlight, thus killing them and get a monopoly on the nutrients. The mat of plants would create a low oxygen environment much closer to the surface. This wouldn't spell doom for all sea life however. Life usually finds a way.
Fish with larger gills would be comfortable in these conditions. Aquatic mammals would be at an advantage as they can breath oxygen and rest on the surface of the mat. The mat itself would also be a food source for grazers, while predators would use the cover to ambush their prey. All in all, a very interesting biome.
However, reality seems to suggest that this isn't possible. At least not at 100%. If this halophyte superorganism were to take over the sea, what various challenges would it need to overcome?
Assume earth conditions (e.g. climate and animals) but no human presence. This plant is naturally evolved.