Boiling is an endothermic process. During boiling, a liquid absorbs heat from its surroundings to transition into a gas. This heat energy is required to overcome the intermolecular forces that hold the liquid molecules together.
When a liquid reaches its boiling point, molecules throughout the liquid gain enough energy to escape into the vapor phase. Since the process requires the absorption of heat, it is classified as endothermic. This is why you often feel the temperature of water decrease when it begins to boil and takes in energy from the heat source.