It is always above the waterline. This floatation is held in reserve. It only generates a lifting force if it is submerged below the water. When a cormorant rests on the edge of a float, the added weight submerges a corner of the balancing floatation. The submerged floatation generates a lifting force that opposes the downward weight of the cormorant.
Polystyrene foam is the only type of balancing floatation used in the floats.
Ballast floatation is unique to the floating island. The mulch in the floating island acts as ballast floatation. When it is below the water's surface, the mulch weighs very little. It would take a large lifting force to raise the mulch free and clear of the water. The inverted buckets under the floating island generate a lifting force, but it is not large enough to lift all the mulch out of the water.