"Turning to your iron fuel tanks, they will be rusting on the inside given water is always present in diesel (hence diesel bug grows necessitating regular biocide treatments when you fill the tanks) and from condensation if tanks aren't kept full."
True BUT,, a real fuel tank (not a box of fuel) will have a sump that is Servicable.
All water that is carried in with clean fuel and any condensation falls (eventually) into the sump where it can be discarded.
The use of a DE Emulsifire will speed the process , should you get a load of water in the fuel tank.
As "bugs" can only grow where there is water , the sump will contain any bug waste .
I am always stumped to why every boats first purchaser does not require genuine marine fuel tanks before passing over that big check.
Or why the error is not noticed on the pre delivery survey.