Are you sure it's the tank itself? It could be a leak around the inlet or outlet tube that ran down between the outer "skin" and the tank and found a weak area along the seam to get out. It doesn't sound like you have enough water to be a ruptured tank, and if it were, you probably wouldn't have had any presure to release with the release valve.
Check the fittings around the top of the tank to be sure. If the installer used steel pipe coupled to copper, it would eventually corrode enought to spring a leak. You may only need to replace the fitting - using brass rather than steel - rather than the entire tank. Of course the damaged skin, and water logged insulation underneath are a problem that might result in a new tank anyway.