The first ( modern ) diving Watch....
We are many, here to not know which is the certain date.
Both Rolex and Blancpain claim that it was in 1953... BUT...
BUT, Rolex unveiled the 6204 / 6200 in Basel, in 1954, and it seems that ( I insist on " it seems... ) that Blancpain did the same, at the same date.
BUT you are right, Cousteau wore a Fifty Fathoms, on the pics from that era, not a Rolex.
Nonetheless, it is hard to tell which one, between the Submariner and the Fifty Fathoms, was the first issued.
Best,
Nicolas