What was a ww1 submarine made out of?

Although the alloy composition has of course changed over the years to provide for more flexibility, submarines of WWI were pretty much made of the same stuff that modern boats are today; good old steel.

Today's submarine hulls (most nations) are made of a flexible steel that contracts and expands with sea pressure, making for a much longer hull life. Some countries, such as China, use GRP (Glass Reinforced Plastic (fiberglass) for their hulls, and others like Russia, use titanium for their pressure hulls. There are advantages and disadvantages to each material.