As with any job around your boat, be prepared for the "law of unintended consequences" to kick in when you least want or expect it. Sure, re-packing a stuffing box in the water is a very doable task, but ask yourself what you'd do if "x,y, and/or z" occurs while doing the job. Have a Plan B ready beforehand. I've found that the most difficult part of this relatively easy task is picking out the ALL of the old packing material. I use a long drywall screw and pliers. I control the inflow of water from the opened gland by tying a piece of old rubber bicycle tubing around the opened gland secured in place by zip ties while working on the compression nut and stuffing the new packing material in there. Make sure your bilge pump works or have a hand pump nearby. I also suggest enlisting the aid of an experienced fellow sailor the first time you undertake this task.