Slugs and snails do not need water to survive. They are able to live without it, but their rate of metabolism is greatly reduced when they don't have access to water. Slugs and snails are members of the phylum Annelida, or segmented worms.
Slugs and snails are generally considered to be a pest because they eat other plants, but some species are actually beneficial, like the milkweed slug (Arionidae). The snail (Gastropoda) is another species of animal that is considered to be a pest.