Looking for water in All the Wrong Places...
In general, old fuel tanks are subject to corrosion on the inside from small amounts of water that collect in the lowest portion of the tank. This is normally at a corner welded seam, usually at one end where three welds come together.
Either the tank has condensation that has slowly accumulated or (much more likely) has been receiving water from the deck fill. If the O-ring on the deck fill is in good condition, there is also a chance that some water was introduced from the source/fuel dock.
At this age, the factory tank is ripe for removal and inspection. If there was no inspection port, this is the time to install one. There is a good chance that the wall thickness of the old tank is below the standard today... and that will complicate welding in a patch and putting in an inspection plate.
Once you have it out, you will very likely be looking at replacement, in any case.
"While you're there" you should replace the fill hose, the vent hose, and the fuel lines to and from the engine too.
When we did this for our '88 boat, we went to a new tank with a baffle, two inspection plates, and an electronic sender for a remote fuel gauge. Note that our original tank had no leaks, but was, IMHO, quite undersized for our boat. There was room for a custom designed new tank with more gallonage and we are extremely happy with that upgrade. That project with photos is on this site.
Good luck,
Loren