Corrosion in Spots
The majority of fuel tanks, especially on yachts, are very dirty by the time filters start to clog, and a problem is noticed.
It is a critical point. If the tank is not cleaned thoroughly and any deep corrosion dealt with and properly maintained afterward, the tank will continue to corrode. Whatever the makeup of the sludge in the bottom of a tank, it is always corrosive. If the tank is made of metal, this is a problem.
If the boat is used a little more than most, it probably needs a good cleaning, but may not have corroded to the point of worry yet.
Usually, the lowest area in the tank will have the worst corrosion. A portion of the lowest area is often lined to prevent any future problems.