Snails and slugs are soft bodied animals which contain considerable amounts of water. As a result, they need to be very concerned with water conservation especially the semi-slugs and slugs who cannot retracted into their shells in drier times. This threat of dessication affects their behaviour.
This behaviour can mean that land snails aestivate [hibernate] in drier times. They seal themselves to rocks, logs, leaves or even other snails. Some are what is called ‘free sealers’ and these secrete an epiphragm which is a calcium impregnated mucus shield [like a door]. The epiphragm is permeable to oxygen but reduces moisture loss. The snail may even secrete more than one epiphragm if the conditions are severe. In these cases, they retreat even further into their shells.
Slugs and semi-slugs who do not have shells or reduced shells have different behaviours. They have evolved so that their activity periods coincide with rainfall periods or the times of the day when conditions are more humid e.g. early morning or early evening. Some slugs secrete very thick mucus that is considered to help prevent water loss. Pulmonates [lung breathers] have a single kidney though much water is reabsorbed to minimise dessication.
Snails, semi-slugs and slugs do drink fresh water.