I think a mechanical engineer or a materials scientist would know the most about important properties of materials.

I bet that not all submarines are made from all of the same materials. Like for most things, we choose materials based on what we want them to do.

I can think of a few things that all submarines need to do. First, a submarine should be watertight: it needs to keep water out, and air in, so that people inside can breathe. Second, it should be able to withstand high pressure. As a submarine sinks deeper, the water around it will push harder on its walls from all directions. We don't want the hull of the submarine to buckle or break. From these things, I would choose a strong, hard material to make the hull, like a steel alloy.

Third, we don't want the outer hull to rust, since it is always touching the water. Rust is made when iron (the main component in steel) and oxygen react, which happens more easily in water. Rust is bad for our submarine because it is a weaker material: it is flaky and breaks apart easily.

We could make a better submarine by looking for materials that are better in some important way. For example, if I found a material that had a higher bulk modulus than steel (that means it's stronger against compression), it might be good for making a hull, and it might let my submarine explore even deeper in the ocean. Of course, I would also have to be able to make it watertight and rust-resistant. I'm sure that there are even more important properties that a submarine expert would know about.

People have been making submarines for more than a hundred years, so by now, I think we probably have very good materials. If you wanted to make it better than what we have now, you would need to have a good idea for how to find a new material (or maybe how to build a submarine). You could make a lot of money doing that, too.