Interplexr, I am afraid the soils there will always be soft, but should settle a little over time since they were never originally compacted. In time, much of the loosest soils will migrate to the deepest parts of the pond as fish move them around plus from wind/wave action.

As for planting spatterdock, are you aware it is a fast spreading, highly invasive plant? It can take over an entire pond in a single season.