As Matt said Oxalic Acid (Wood Bleach) will work great! Then take some light polishing compound with a buffer or just go right to a Polish/Wax. You will want to seal off the pours after you use the Oxalic. Wear gloves while sponging on. You can dilute down to where it will take about 15 minutes for the stains to come out. You do not want it strong enough to wipe on and see the stains coming out as you sponge it on. That is just too much. Take it slow. As you are sponging think about seeing results as you finish going around the boat and seeing the first part changing. Go around again until you get the desired results. May want to wash with soap and water before waxing.

Later,
Dan