Waterjets can cut just about any material that can be made into a sheet.
The most popular materials are metals (especially aluminum, because it’s relatively soft and cuts quickly), because waterjets can cut intricate shapes to a high precision quickly and economically. Since metals are the most common material cut by machining shops, waterjet cutting is very often the chosen cutting method.
Waterjets also commonly cut stone and glass, because the waterjet can cut intricate shapes not possible using traditional machining methods. These materials are often used where aesthetics are important as waterjet cutting allows almost any shape to me cut.
Among the very few materials that waterjets cannot cut are diamonds and tempered glass. Diamonds are too hard to cut, tempered glass will shatter when it is cut with a waterjet (tempered glass is designed to shatter when it’s disturbed and is frequently used in windshields for this very reason).
A few advanced ceramics are so hard that it’s not economical to cut them. Some composite materials (layers of different materials sandwiched together) can’t be cut because the water can seep between the layers and “delaminate” the material.