I had the same problem with my Columbia T-23 when I purchased her last year. I took down the mast and removed the deck mounted mast step. I found that the original sealant under the mast step was bad and letting in water into the cabin top core for a long time and had rotted out the core. There is a plywood beam within the cabin top which takes the mast load and this was in good shape. But with the core going soft it created a depression under the mast step which just let in more water. The solution I used was to drill a series of 3/4" holes around the cabin top near the limits of the bad core (ended up being about a dozen holes) and hook out the rotted balsa core with a heavy wire and a hacksaw blade. I then filled the void, including the drilled out mast step bolt holes, with epoxy and this gave a nice solid top to remount the step. It took about a day to drill out and remove the bad core and a morning to epoxy fill the void. I allowed about two weeks under cover after removing the core to help dry out what was left. This spring I plan to sand out the cabin top surface and paint it with non-skid paint. This will make the repair almost un-noticable. (I was planning to repaint the deck anyway) Good luck with your repair.
