Unless I just can't see something, there's not enough weight to hold those in place. I've been down the same road.
We tied wire around everything and tied that wire to cinderblocks. And everything else not attached to cinder blocks weighed down some other way. I’m also planning on a lot of this to get water logged.
There's a lot of force from rising water and wood that still wants to float, most of them will be dislodged and floating before the pond fills. I thought the same thing, only to see everything floating randomly around the pond. If you have some earth anchors, they work well and are cheap.