Underwater cutting is a common practice in the offshore diving industry, however, acetylene is not used as this becomes explosive below 6ft. There are a few different methods used. The most common is the 'oxy arc' or 'thermic arc' cutting rods. These burn at a temperature of 10,000 degrees f. and will cut through anything, including all metals, concrete, rock etc. The technique uses a DC welding machine to create an arc, an Oxygen supply to create the burning environment, both connect to a torch via a hose and welding cable. This torch host's the cutting rod, this is made up of mild steel rods and a magnesium rod encased in a copper tube. Oxygen is passed through the rod at a pressure of about 90psi over bottom pressure, the current is applied to the rod top create an arc and ignite the rod ( a stricker plate can be used for this). Once the rod ignites, the current can be turned off. the rod will continue to burn as long as oxygen is applied to the rod. This requires a fair bit of experience to master, and is expensive, and requires a lot of equipment. to see what these rods look like go to the following URL. http://www.woodtech.force9.co.uk/ucr.htm