The whole area both sides of the river, including the river its self, sits on a girt big shelf of Sherwood Sandstone...and you haven't got to go too deep before you hit it...Woolton, Thurstason, those tunnels as you come into Lime street, etcHas anybody ever built a stadium "on water" before?
In one of my previous incarnations I did Industrial drain cleaning, dirty but lucrative, I got called to a chinese chippy near the nnottom of Stanhope st by the queens dock, pulled the drain cover in the street and the drain was there cut out of the sandstone...we got free dinners off the guy too.
I would guess that those dock walls are anchored to it... I doubt something of that size just rests on some sand and river mud. The big problem will be when you remove the counter balancing weight of water from inside the dock. In engineering terms its not new tech that has never been done before, essentially just a dam.