Not all shipping labels are waterproof. Although most shipping labels have some level of resistance against moisture and splashes, they are far from being waterproof. This is because most of these labels are paper-based. However, there do exist labels that are completely waterproof as well. These are plastic film based thermal labels. These types of labels do not contain any components that can absorb water. That being said, you cannot accept such labels to stay stuck on a surface if it gets too wet for an extended period of time. The adhesive will slowly dissolve away in the water, and the label will come off.