If you are looking for free floating sea vegetation, go for the Sargassum
Sargassum is a genus of brown (class Phaeophyceae) macroalgae (seaweed) in the order Fucales. Numerous species are distributed throughout the temperate and tropical oceans of the world, where they generally inhabit shallow water and coral reefs, and the genus is widely known for its planktonic (free-floating) species.
Most species within the class Phaeophyceae are predominantly cold-water organisms that benefit from nutrients upwelling, but the genus Sargassum appears to be an exception. Any number of the normally benthic species may take on a planktonic, often pelagic existence after being removed from reefs during rough weather; however, two species (S. natans and S. fluitans) have become holopelagic—reproducing vegetatively and never attaching to the seafloor during their lifecycles. The Atlantic Ocean's Sargasso Sea was named after the algae, as it hosts a large amount of Sargassum.
They are also known for supporting micro-habitats:
Large, pelagic mats of Sargassum in the Sargasso Sea act as one of the only habitats available for ecosystem development; this is because the Sargasso Sea lacks any land boundaries. The Sargassum patches act as a refuge for many species in different parts of their development, but also as a permanent residence for endemic species that can only be found living on and within the Sargassum
EDIT After the OP mentioned the plants have to float above water
Example of floating vegetation islands are found around the world. For example, in Italy there is the lake of Posta Fibreno (referencing the Italian page since the English one doesn't report the same content) hosts for centuries an island made by peat and small vegetation growing on it, with roots directly dangling in the water.
However, open ocean has the problem of the lack of fresh water, therefore you need to adapt using plants which can fare well using sea water, like mangroves.