I don't think these can be started in water. BUT I have started them in damp spaghnum moss. As long as it is not dripping wet it should be ok. These are succulent and already store water in their "stems".
The best way to start them however is just lay the cut end (after you let it callous, or let the cut end dry for about 24 hrs) on top of your soil and gently nestle the cut end into the mix. Water sparingly. Until they are actively growing they will not need regular watering.