The balsa that you would use to build a model airplane is not what you would use in your deck. The coring material used in most if not all boats is made by a company called Baltek. It is high density end grain core balsa squares on a fibreglass mat. It is not expensive and is readily available.
Alternately you can recore using a closed cell foam such as kleg cell. It will not wick moisture or rot however it is not quite as dense and on a large flat area your deck would be slightly springy.
If using balsa make sure to seal up any through holes via the epoxy potting method or it may just get wet again.