The only time to add water to a roaster oven is if it is intentionally being used to steam food. Adding moisture during the roasting process can prevent the skin on meat from browning and crisping properly. It also dilutes the natural flavors of meat juices.
A roaster oven should come with a manual that includes directions for using it and other information, including the safe internal temperatures of meats and how much water to add when steaming foods. Steaming vegetables and other foods requires a separate steaming basket or tray, and oven mitts should be worn when removing hot steamed foods so hands do not get burned.