I'm reading a technical diving book called "Deco for Divers," by Mark Powell. In one section he writes the following:
Siebe Gorman...introduced the technique of using different concentrations of oxygen mixed with nitrogen. This was used to great effect by British commandos during World War Two who used oxygen-enriched air rebreathers. Their rebreathers used mixtures of 45-60% oxygen which had a greater maximum operating depth than their opponents whose pure oxygen rebreathers were limited to 6-10m. The British used this to their advantage and their simple but effective strategy was simply to pull their opponents to deeper depths until convulsions overwhelmed the enemy diver.
I find it hard to believe that enemy divers regularly ran into each other such that a regular strategy developed, but that's where my question lies. Did these encounters actually happen, and how would these engagements come about? What do we know about them?
I'm highly sceptical about the quoted text. Most British diving efforts during the war used variants of the Davis Submerged Escape Apparatus (DSEA), a rebreather developed in 1929 for use in escaping from submarines. This is described in all of my sources as using pure oxygen as a breathing gas. The only place where I know British divers engaged their opposite numbers was Gibraltar. The Italian Decima Mas(the Italian navy's special forces organisation) used the nearby port of Algeciras as a base for attacking shipping in Gibraltar. A house, the Villa Carmela, was purchased for use by combat swimmers of the Gamma unit. The tanker Olterra, which had been scuttled in the harbour, was converted into a secret base for the Maiale human torpedo. From these bases, the Italian frogmen would venture out to place limpet mines on the hulls of ships in Gibraltar harbour. The first such attack provoked a British response; Lieutenant William Bailey, the Render Mines Safe Officer (the RN officer responsible for bomb disposal in the harbour) formed an Underwater Working Party (UWWP) to defuse the limpet mines and to protect ships against attack. Bailey was soon joined by Lieutenant 'Buster' Crabb, who would later become famous for mysteriously vanishing under a Soviet cruiser in Portsmouth harbour, as well as three petty officers. The UWWP was mostly concerned with bomb disposal, defusing and removing limpet mines. On only one occasion did they come face to face with their Italian counterparts. In July 1942, while Bailey was investigating a ship in the harbour, he encountered an Italian frogman. Bailey attacked the Italian with his knife, but to little avail; the Italian was able to evade him as the British did not use flippers, while the Italians did. The main defence against swimmers and human torpedoes were patrolling launches armed with depth charges. These successfully prevented attacks against Gibraltar and Alexandria on a number of occasions. In May 1942, depth charges foiled a human torpedo attack on Alexandria, with six divers captured before they could lay their mines. On the 7th December 1942, three Italian divers were killed and another two captured following depth charge attacks on an attempted human torpedo attack on Gibraltar.