Todd if your looking to build an island with plenty of plant roots extending into the water, using plastic fence mesh worked well for me, I covered it with newspaper then put a mixture of peatmoss and potting soil on it, the media on it has filled with roots and held up well after almost 4 years.
The flexible plastic pipe filled with foam and swim noodles has provided enough floatation, but if you were to build a large island 4 inch corrigated pipe could be used to shape the island then drill holes in it to fill with great stuff expanding foam, this would probably give you enough floatation.
After zip tying the plastic mess across it add small pvc pipe bracing to help hold it's shape.
The christmas garland I used was purchased at the Dollar Tree store, the stem is metal coated in plastic and the pine needles are also plastic, it also has held up well and there is no need to weight it as the metal makes it sink.
Just some thoughts on contruction, there's plenty of ways to experiment, good luck.
http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=312455&page=3
