Just to clarify - these tanks were definitely at Normandy. There were three tank battalions attached to the assault waves, the 70th supported the 4th Division at Utah, the 741st and 743rd supported the 1st and 29th Division forces at Omaha. Each battalion had three medium tank companies and a light tank company. For DDay each battalion had two companies equipped and trained with the M4A1 Duplex Drive tanks - these were the Shermans with an inflateable canvas "duck boat" that could propel themselves in water. The third company had their tank equipped with the "snorkels" and was to be delivered straight onto the beach. This company was reinforced with extra tankdozer M4 types. The light tank company and most battalion service elements were to land late on DDay or D+1. Further, a number of the initial follow on units had their tanks equipped to land in deeper water - they came in from LSTs and LCTs before the Mulberry piers were assembled. After the ports were established, or French ports captured, the tanks didn't need the extra fording gear so you don't see the stacks on later units' tanks.
Gary