Making an Island/Plants
Plants are the most important part of a floating island as it will be extremely prone to erosion. You will have to make sure the plants you put on the island can grow in whatever environment you chose to put the island in. If you build your island in saltwater, you might need to use plants that can survive in environments that are rich in salt, such as mangrove trees. If you are growing plants that are not resistant to salt water, you need to make sure that the soil in which the plants are growing has no contact with the subterranean sea water. You might want to cover the bottom of the surface which is supporting the soil with either concrete or wooden boards.
Additionally, the roots of plants will add support and strength to the soil. Grass might be a good plant to start with, which will also improve the aesthetic condition of the island.