The tabs are packaged in individual plastic tubs with foil lids, kind of like you sometimes see for margerine packs at a diner. They seem waterproof. The tabs also have some sort of water resistant wax or coating on them. If you take a tab out of the package and douse it with water, it'll light with a match or lighter. I just tried this with 10+ year old tabs and they worked fine.

Once a tab has been lit and extinguished, however, it's not waterproof anymore. They seem to absorb water and are very difficult to light. Also, while it's possible to light a tab with one match, it took me two. It's easy with a lighter, or with 1/2 of the spark-lite tinder.