I didn't realize that yours is due to lack of clearance between tank and deck. I don't think that is my issue - there is a decent amount of space between top of fuel tank and the deck.
I believe that sag in ploy fuel tank is actually common. If you notice in the wema fuel sender instructions, you need to order a sender that is shorter than the tank depth by at least 1" - I believe that this is to allow for tank sag - if you have a metal tank, they say you only need 1/2".
I wonder how big an issue the sag is anyway. If you are cruising along and water gets down there, it's not really going to stay on top of your fuel tank, especially if you are hitting any waves, even small. I would guess the main time it would be an issue if when it rains and the boat is sitting. If the boat is covered, then it is not an issue.
I suspect that my water issues came from slow seepage over 10+ years with some time sitting on a mooring. If I keep an eye on it and clean the tank ever y couple years then I am hoping that will head off any problems. Cleaning the tank seemed like a big deal before but doesn't phase me now - just run the tank down as much as possible and pump the rest out by hand.
