Yes, it would have to be from the water side. The pressure would tend hold the repair in place instead of pushing it off. Don't forget, when the steel heats up and cools down, the crack will grow so it is a temporary repair at best. To repair it properly, it will need a hole drilled at each end of the crack to relieve the stress and then the crack would be Veed out and the entire thing welded by a competent welder with at least three passes.
If the boiler is "H" stamped (rated for steam), the repair must be performed by an "R" stamp holder to maintain its rating. Not a big deal to the homeowner, but it is a very BIG deal to Factory Mutual, Hartford Steam Boiler and other insurers. It will also be a big deal with your state's boiler oversight dept. If this is a commercial installation, you must get an "R" stamp holder to make the repair.