Andrew, I had tried that approach originally but found that it was not possible to contain a floating bio mass similar to what you talked about. With the method I ended up with you are able to contain the bio mass in an area and as such not fill the pond totally with plants. It also allows for root growth under the island and as such has fish around it at all times. Here is what it currently looks like.
As you may be able to see in the second picture I have water Iris and some water hyacinths on the pond. The hycinths will take over if you let them, but are also great for floating cover and roots to hide in. My Iris are on the sides where I can contain them fairly well.
from a distance:
