Neil, about all I could think to add to your system that may be of value would be a "bottom of the tank tube" to literally pump the water/??? from under the pick up.
I actually sucked the bottom of my ski boat tank every other year. I had to do this via a syphon and always got a bit of water. This was in a boat used on the Co. River in the desert. I even removed my pick-up tube on Volador 4 or 5 times in the slip with an old "Auto pulse" pump I had set up for transferring fuel while houseboating.
I also had good luck on the ski boat because I used a water separator (Fram) and then a filter/separator.
Dave Neptune