According to an article in Boat US magazine, trying to mix old gas and new gas only gets you more stale gas. If you can pump the tanks down about 15-20 gallons, you can pull the sending unit and siphon the gas out. I used a sealed electric fuel pump and a 3/8 hose. I ran the hose thru a 3/4" PVC pipe so i could control the hose better. If your boat is like mine and the Aux tank sender is in the front of the tank, you'll need to drop the bow as low as possible to get the water to run to the end where the sender is located. You may be able to save the gas, I know how expensive this gets BELIEVE ME I KNOW. Trying to save the old/new gas mixer isn't worth the trouble unless you can find a biz that can polish the fuel and guaranty the octane hasn't been compromised. Lowered octane can cause pre-ignition and damage the engine(s). I would also clean ALL the fuel lines, filters and blow out the carbs. The fuel lines can hold eneough water to completely fill the float bowls with nothing but water.
I went thru 5 sets of filters trying to "Short-cut" this ame problem and STILL had to completely drain the tanks and lines. In my case, I had to drain off 140 gallons of fuel and get rid of it. Check around. Maybe someone in your area that blends oil/gas, transmission fluid for a "trash burner". The guy that took my fuel did just that to heat his HUGE shop.