I did the same and replaced my shelf on 23ss.
I used 1/2" Oak plywood. It was $45 at home depot.
I also double coated it with polyurethane on both sides and all the edges to help protect it from any further water that may get on it.
I also added some "Hearth Kushion" to the edge of the shelf as the wife was complaining of the sharp edge entering and exiting the bunks. Found some at home depot but cannot find it on their site but found it at target website.
Target:KidKusion Hearth Kushion - Brown
The factory shelf is the junky paper board as I call it and it does not take much water at all to ruin it. Spilling a drink on it could ruin that stuff.