One hand for you, one hand for the boat
There is an old saying in boating: “one hand for you and one for the boat.” It can be is a critical maxim to keep in mind. We all enjoy a day out on a boat, but so often there are things that have to be taken care of. It could be something as simple like as bringing in the fenders, or something more urgent such as tying down something a line that has come loose. Short of boating in your bathtub at home, there is always the chance that the boat will suddenly lurch, turn, etc., while you are taking care of whatever task is at hand. It could be that you are in the ocean, or worse yet going across a bar where a river and the ocean meet, or it could be in a calm sea or lake when the boat suddenly has to make a sharp turn to avoid another boat or something in the water. Whatever the cause, while you are taking care of whatever it was, you are suddenly at risk of being thrown around the boat, or, worse yet, off the boat.
So as you as you move about on a boat walking around a boat, always use one hand to do whatever it is that you are doing, but always use the other hand to secure yourself to the boat. When you need to use both hands, first look ahead to see if the seas are going to hit the bow, or see if the way ahead looks clear, then use both hands quickly so that you have a hand available to hold on. Better yet, loop an arm around a lifeline, stanchion or other fixed item while you use both hands. You cannot fetch the fenders if you have been thrown to the other side of the boat or over the side!
Hope for the best, but plan for the worst.