Final answer:
Signal for help is the priority action if you fall overboard without a PFD, followed by clinging to the boat or any debris. Treading water and swimming to shore are less preferred options due to the risk of exhaustion.
Step-by-step explanation:
If you fall overboard and are not wearing a Personal Flotation Device (PFD), the best immediate action to take is to signal for help. This can be done by waving your arms or shouting, although it is a myth that a drowning person can always do this effectively. If you are close enough and able to do so without exhausting yourself, clinging to the boat or any floating debris, such as a log, can help you stay afloat until rescue arrives. Swimming to shore may be an option if the shore is close and you are confident in your swimming ability, but if it is too far or your swimming skills are not strong, it could lead to further exhaustion or danger.
Treading water is another method to stay afloat, but it should only be done if rescue is imminent or you're waiting for a better opportunity to ensure your survival, such as the boat coming back for you or getting closer to debris you can cling to. Remember, conservation of energy is key, and staying calm will help you float more easily and increase your chances of rescue.