Not much is well known about the X MAS swimmers.
The X MAS "Gamma" assault swimmer did not "walk" to their targets, they did swim. The walk technique was tested (an idea of TV Eugenio Wolk, born Evgenij Wolkov, and later "father" of the Argentine Navy Buzos Tacticos), but it simply didn't worked: the "walkers" soon get exhausted by trudging into the deep mud in the harbors' bottom (the longest "walk" Wolk reported was just 400 meters). So they switched to swimming, with *much* better results.
And the italian limpet mines didn't had exactly a timer. They had a small screw (clearly visible in the picture), activated by the ship own motion, that detonated the charge after a pre-set number of "turns", so the explosion would occur in open sea, where the target ship would have been unable to beach or move to shallow waters to avoid sinking. That was absolutelly necessary, as the charge was actually rather small. An additional conventional timer was soon added, to delay the activation of the screw by a variable time, to avoid that the screw could be moved by sea currents while the ship was still moored in the harbor. All that required a fair amount of intelligence work, to know as closely as possible the time of departure of the ship and its destination, so that the "time timer" could be set to match the approximate date of departure and the "screw timer" could be set to the farther distance from both departure and arrival harbor. In that Lt.Ferraro was greatly helped by the excellent work of Lt.Giovanni Roccardi, a brillant undercover navy intelligence officer working, like Ferraro, at the Italian Consulate of İskenderun.
The Regia Marina gets a lot of shit in the history books but there is no disputing the sheer size and brassiness of the pairs that their frogmen sported.