In the scene where Bond is trapped in Largo's shark-infested pool, he uses his mini air tank to breathe. In most of the subsequent shots, look closely at the actor's face: it looks a lot more like a stuntman than Sean Connery.
If you noticed that almost all of the bad guys including the villain are all Italian, this is no coincidence. The original story involved the Mafia and the villain Emilio Largo as a Mafia chieftain. The story was changed to replace the Mafia with the organization SPECTRE.