We can’t quite claim to have invented this idea because it is mentioned here.
But it is a rare and noble variation of Pooh Sticks (which we thought of too) which we think should be developed.
We played it with people of Bristol as part of

We were throwing sticks of the famous Banana Bridge over the Avon New Cut, to get people to see just how fast the tide was flowing – and which way!! Ebbing?? Flooding??
Now a variation of tidal pooh sticks we want to develop is a longer version of the game. But we have still to test this. If one picks one’s moment – using, of course, a tide timetable, one could throw a stick into the river , watch it float with the tide under the bridge as usual, but do this soon before the turn of the tide, and then maybe in a while it will float back the other way!