check the top of the tank for pin holes and a deck leak that allows water to sit on the top of the tank. I would also check the fill fitting where it is welded to the top of the tank. The welding process makes that a likely spot for the weld to fail over time. If you get to the point of replacing the tanks, American Tank Company in San Diego is well worth the phone call. He travels all over the country replacing fuel tanks on boat. He replaced mine for 3K when the next closest bid was a little over 10K. The guy is a magician and you will save money on the over all job even paying for his flights in the mix. I like the idea of a day tank to get you down the road. You can transfer fuel to the day tank from your leaking tanks and really monitor the fuel while doing it. If it was me, I would abandon the built in fuel tanks completely and not risk my engines until I figured out where the water was getting in and had it fixed or replaced.