Yes, a slow cooker can be used as a water bath for sous vide. A slow cooker can be filled with water, and its heating element will be used to heat the water.
However, a slow cooker may not provide as precise temperature control as a dedicated sous vide machine, and the water may not circulate as well hence you need to monitor the water for temperature changes.
It's crucial to make sure that the slow cooker is large enough to accommodate both the food and the water, and that the lid seals properly to prevent evaporation.