Not all mattress protectors are waterproof, but many of them are designed to be waterproof or water-resistant. A waterproof bed cover or waterproof mattress protector is specifically designed to prevent liquids from penetrating the mattress and causing damage. However, there are other types of bed protectors and mattress covers that may not be waterproof but still provide some level of protection against spills, stains, and allergens. For example, a mattress cover with a zipper may not be completely waterproof, but it can still protect the mattress from dust, dirt, and other debris.