The story looks like an urband legend: the 60's would mean more than 15 years after the war and I would expect that something would have given in to let enough air out and enough water in to eliminate the small positive buoyancy well before then. The russians as I understand have to make active efforts to prevent their cold war era decomissioned boats awaiting demolition from sinking at their moorings; some abandoned ships have remained afloat for years supposedly but I am not sure that long and with much more positive boyancy to start with.

Anyway, U-166 seems the only boat sunk in the Gulf proper, wreck discovered in 2001. Some others may have sunk at the edge of the Gulf and the potential wildcard of a boat not being where it was supposed to be cannot be ruled out.