Salt water or fresh? If it was salt the motor is absolutely toast. As mentioned the bearing surfaces will be etched from rust and will fail eventually if you did get it running.
If it was fresh water there is a remote possibility it can be salvaged. But, it's been sitting for a month so the powerhead will need to be torn completely apart and any rusted or corroded bearings, pistons, crank journals will need to be repaired or replaced. Likely will need cylinders rebored or at least honed. That means new rings and possibly pistons anyways.
Unless you are going to do all the labour yourself it will cost more to repair than it would to replace.