Fresh Water Tank Leak

Bladder won’t work because the tank most likely has baffles inside... if the hole is small one try to clean any corrosion, rough up the stainless and use thickened epoxy to patch it. If the hole is too big they may need to use some glass. I wouldn’t do that with a fuel tank but with water they don’t have anything to loose.

The better repair would be to find, access and weld repair the leak.
A patch from the inside with food grade sealant would be the next idea.
Where is the boat?

If the leak is a the top... sure. But if it s at the bottom can you safely weld against a fiberglass floor?

It takes a fair degree of luck to reach the exact leaking spot with an endoscope, inside a baffled tank.
Still, depending also on how reachable the hose is, it might be worth a shot.

Anyway, I wouldn't feel very optimistic on the overall conditions of the tank.
Sounds like it wasn't built with the best stainless steel available, to say the least...
That said, if it really didn't get any worse in three years, I'd be tempted to leave it as it is and keep it monitored.
Judy, any chances that corrosion actually started from the outside, for some reason?

Welding against a hull member or insulation of course would not be a good idea. A good examination of the area would be required here.
A plan could involve even a partial de-install of the tank just for better and safer access.
A shipwright and metal fab tech would be able to investigate this course after inspection better than we can from our keyboards.
An after factory weld needs to be considered a patch also. The tank will always be concern in the future.

I was thinking a patch from the inside, cloth or a metal scab over it could help. Then the water weight would always be pressing a tighter seal.
From the outside, I would use rivets holding a metal scab and sealant in place.

I agree.
On some boats, the boat is built around the tanks. Probably why Judy opened this thread.
As we have offered before, cut it out and install a few smaller replacement tanks tied together.

There may be a reason for the failure other than the tank; Poor install on its bed or frame.
In my boats, I have replaced a few bad tanks. Poor installs were half of the failures.

So the weep is under the tank? Then it has to come out... but it’s all speculation without knowing where the tank is.

I ve seen stainless steel water tanks under beds where it may just be a matter of cutting the bed to get the tank out. Usually the problem will be to get the the tank out of the room and boat if you want replace. In that case if the tank can be inspected and repaired it maybe the best option. Otherwise cut it up to get it out and replace by two smaller tanks that will fit in.

it all depends on the boat.