They can be. Unfortunately, a more complicated answer is required.
If a sticker is die-cut, it doesn't mean it's waterproof. Die-cutting refers to how a sticker is cut, not the material.
Some materials are waterproof, and some are not.
Plastic materials tend to be waterproof, and papers tend not to be waterproof.
Find out more information about each material here.
For best results, ensure your sticker is laminated to protect your printed layer from getting washed or rubbed off.