Final answer:
In an emergency underwater at night with no communication, flashing a light is the best signal to attract help.
Step-by-step explanation:
In an emergency underwater at night, if communication is down, the best signal to give for help would be to flash a light. Flashing a light is a visual signal that can be seen from a distance and can attract the attention of others. This is an effective way to communicate when other forms of communication, such as radio, are not available.