If you don’t plan to use a hydrocollator for an extended period, you should remove the water, sanitize the unit inside and out, and leave it open to dry thoroughly.
Hydrocollator packs should be stored inside the unit while it is full and heating. Some packs are filled with clay or other materials that, when dried out, become ineffective and need to be thrown away. If you don’t plan to run the hydrocollator to keep them bacteria-free, then simply make sure they are saturated, place them in a plastic bag, and freeze them until you are ready to use them again.