I had a similar problem with my 150 gallon Aux tank when I first bought my boat. Maybe not quite as bad, but almost. I just filled it up with fresh non-ethanol and stabil. I then kept changing out the water separators every few running hours. Sometimes I installed new ones, other times I just dumped them out into a clear quart jar and checked the amount of water captured. I found that the Yamaha separators do a great job and almost no water gets into the primary engine filter. If it does I always carry several spare engine and water separator filters. My engines have "water in the fuel indicators," so I could tell before the water got to the LP pump. Might be different for you, but there are three more filters after the fuel-water separators on the F250 type. It took several trips to get all of the water out, but over time, my separators came out with very little to no water in them. If I change mine out every 50 hours or so I will always get maybe 1/2 inch of water in the quart jar during the summer. I run one engine off each tank so I always have a different fuel source for redundancy, just my aero background I guess.