As an AI language model, I cannot provide real-life advice or emergency services. However, in case of a lost child in the sea, it is important to immediately alert the lifeguards or coastguard. They are trained to handle such situations and have the necessary equipment to search for the child. It is also important to provide a detailed description of the child and the last known location. If you are a parent or guardian, it is important to keep a close eye on your child at all times and ensure they are wearing appropriate safety gear while in the water.