Personally I'd stick with marine ply under a deck stepped mast. Were it my own job, I'd probably run one piece the thickness of the core to replace the balsa with a generous lap, say two or three times the area of the actual step. Another piece of 3/4, cut to the shape of the step with the edges chamfered would provide a pedestal of sorts to avoid the potential for water to find its way in.
Again, it's your project so it's your call, but balsa isn't commonly used in area where there's point loading like under cleats and winches but is 'fine' in areas where the entire panel is loaded (like side decks, cockpit seats, etc)
"Verbosity leads to unclear inarticulate things"