It appears that it was sitting in about 3 inches of water at some point. It was mostly swelling and very brittle at the bottom. A few chunks of MDF came off when moving it around.

With a little guidance from
Randy Fromm, I sanded down and cleaned up where the damage was. Then I used Body Bondo to fill in the chunks and all over the damaged area and sanded that down once it was dry. You can still tell there was swelling on the left bottom side but it came out quite nice.