The first thing you must do is this;
if the boat is still floating after the accident and you are still aboard the boat, then find out if you or anyone involved in the accident need medical attention and administer first aids. Keep the boat out of harm's way when doing this.
If the boat overturned and someone fall inside the water, help the person[s] to get inside the boat again if the boat is still intact, if not get on a floating debris and signal for help.