A water lock on your washing machine protects against water damage when the washing machine starts to leak. Therefore, check whether your (new) washing machine has a built-in water lock or not. Are you buying a second hand washing machine? Check if the water trap is still intact. A small damage to the water seal can get out of hand! There are 3 different water locks: a bottom sensor, hose lock and overflow protection. The bottom sensor is a built-in water tank, the hose protection stops the water supply and the overflow protection pumps the water away again. Ideally, you will have all three of these water locks connected to your washing machine.
