JC Whitney has an inner liner that I have used very satisfactorily in repairing pitted and corroded gas tanks on antique vehicles. Get the literature and find out if its satisfactory for potable water. Difficulty is that the tank must be removed and if you've got it out you may as well either weld a new bottom on it or replace it.
Another alternative (preferred IMO) would be to install a plastic bladder in the existing tank. The baffles will present some engineering challenges but there are several advertisers (Good Old Boats Magazine) that can custom build plastic tank liners.
I replaced mine with an off the shelf 80 gal plastic and after a fun afternoon dealing with epoxies and fiberglass while standing on my head had it both securely mounted, and easily removeable.
Be careful, You can get a cheap(?) high from the various solvents etc. found in dealing with epoxies or polyesters. This may not be conducive to becoming an old sailor!
Seafarer 29 KCB