A ten year old girl was being towed behind a boat on an inflatable tube when she fell off, and was struck by the propeller on her right foot. Even more significantly the boat was in neutral, and the injury occurred when she kicked her right leg, and it made contact with the propeller. The injury was traumatic enough to require surgery.
What this report shows is that you can do everything right when boating, have the boat in neutral, and still a traumatic bleeding injury can occur. So you should always carry a maritime first aid kit that contains marine specific first aid equipment. The water is a different environment, and traditional land medical devices don't meet the specification requirements to get the job done, especially tourniquets. At a minimum you need at least one marine tourniquet.