Here is my experience with a floating island. The island had about 6 inches of soil with ropes drawing water up and into the soil so it was always moist. The flowers did just great. But roots cannot just dangle in the water because they became coated with particles, become black and the plants die. The soil filters the roots.
Now you see cabbages planted with one plant fertilized, one plant planted in pond muck and the others just left to their own devices. The fertilized plant looked great for a while but all plants eventually died. I have no idea why. But the flowers as I said did great.
