I did use those supports. They will "float" on clay and travel, so you will need to pin them to the substrate somehow.

What I did is dug a 1' hole where I was going to place these holders, pounded in 3' long masonry spikes so only 6-8 inches was remaining, then ran masonry wire between the holes of the spikes. Then I filled the hole just to the top of the spikes with concrete, then set the pre-formed post holders with pre-attached metal brackets for holding the 4x4s to the block. I forget what the brackets are called, but they are basic U-shaped brackets for decks with stainless masonry anchors in the block to hold it in place. Maybe overkill, but I was putting these on the slope of the dam which was full of stone and clay and had not settled yet.

I was able to see them two days ago as my clarity was excellent and they have shifted a bit as things settled under water. I am anticipating building up the dock soon as we are supposed to get unusually warm weather and I am doing to jump in with a wet suit and have at it!

I wanted something a bit more secure since I am expecting to install a small diving board, and I don't want my dam scooting around the pond.