My plan is to anchor it, with a (say) five foot long cord. The reason being I want it to swing with the wind, so the plant roots don't establish a grip on the pond bottom.
Today's launch was a sort-of concept test. (I had hoped it would float, rather than sink.) As I but dimly understand it, once the acorous and pontederia roots tie the bark chips together, their root mass will generate some gas that will co-generate buoyancy. If that happens, then I may be able to remove some of the PET soft drink bottle floats.
I may be persuaded to construct another floating island. Consequently, I'm wide open to suggestions for good nutrient-stripping water plant varieties, that can cope with ducks and are available in New Zealand.
By-the-by, I have heard a frog croaking away, in the last few weeks. Last time I heard that was back in 2017. It's always a source of wonder and perplexity to me as to how Nature 'seeds' things. Last I knew, neither tadpoles nor frogs had wings.
Recall that, "Nature finds a way" remark by Sam Neill in Jurassic Park?
Those two fly-in-invaders and 'seeding'' mallards being yet another example.
