Don't be so certain--my experience

My experience was on the McCloud Railway. The Bartle Water tank, looks at least as large as "yours" was in similar condition. we straightened up the tank, tightened the bands some, then set a sprinkler in the tank. The wood swelled up, and held water!!! Before that, you could see between the cracks, just as you have noted. Unfortunately, I understand that the wooden water pipe to the Bartle tank failed, and the tank is once again dry.
The hard part we had to deal with was that some of the staves had started to collapes inward We had to use deck screws and straps over the bands to pull the staves back out. We also had to use them to keep the bands in place until the wood swelled up enough (actually, we left the screws in. Then there were the bullet holes, doweling filled them!
IMHO, keeping water in a tank is one of the best ways to preserve it!
Steamcerely,
David D.