There are many different types of alloys out there to use. Practically thousands! So, to say that one alloy is better than another just depends upon its properties of the metal. These properties are defined by the composition ( which can sometimes be pretty complex), and the fabrication means. The composition varies only slightly to change one alloy from another sometimes only by 1/10 of 1%. This is not very much change in composition, but it can cange the properties greatly. The fabrication is also a very important factor. If the metal has been heated in any way, rolled,welded, or even drilled in some cases; then the metal has residual stresses in it. This metal then needs to be "stress relieved", which just means that the whole thing is heated up, and gradually cooled of depending on what properties are desired, and what alloy you are starting out with. There are companies that do such stress relieving. They are the best ones to contact on which procedure to use for which alloy. The main key to asking which alloy is used for depth probably depends on Tensile strength. This is rated by how many pounds per square inch it takes to pull the metal apart. To give you some idea, standard metal that you might buy at a local steel supplier has generally about 30,000 pounds per square inch (psi) of tensile strength. A standard 7018 welding rod that is used to weld it together has about 70,000psi. The real submarines and some very expensive personal subs that go very deep use an alloy called HY-80, which has about 80,000 psi. I saw on the Discovery Cahnnel that they are now using an alloy called HY-100 steel, which as you guessed has a tensile strength rating of 100,000 psi. The only thing is that the stronger your material is, the more expensive it is... I hope this helps you.