The boards probably have a feather down the length on each side. The right way to do it is to drain, loosen the bands, pull the bad board, cut out and replace the bad part.
But if it has been there a while, that might be tough.
How about screwing a patch over the inside with SS screws? Obviously much easier if the tank is open on top!
I suppose you could screw a patch on from outside too. But then water pressure is working against you.
Else-wise, if you can't get at it, drain down, drill out the bad part, and cut so that you have an oval hole. Taper the sides from inside, so inside is bigger than outside.
Ct and fit a tapered patch, which you can fit thru the oval hole like a handhole cover on a boiler goes in. Put a piece across like a strongback, with any sort of sealant you wish to use on the patch, and tighten down..