If the tank is sitting in bilge water, chances are it will be a problem. If it's a fresh water boat, maybe less so.
The aluminum fuel
tank on my E42 lasted a little over 20 years with the packing gland drip running onto the tank and sitting in other bilge water as well. PO replaced it just before I bought it in 08. I'm just about done getting all water sources stopped. So I should be good for more than another 20 years.
On my boat, estimates have ran as high as $60,000! That was BS. That estimate chose the hardest way possible to replace the tank, by removing the entire engine. It was done for much less than $10,000 with a new stainless steel tank.
One way of checking condition is to try to get a sample of the fuel
from the very bottom of the tank and see if it's clean. That won't necessarily tell if the tank is leaking but if the fuel
is clean and water-free then the tank likely isn't leaking. If there is crud or water, it may or may not be from a leak.
Hard to say just how easy it would be to pull that kind of sample.