Sheathing would be the equivalent of a Band-Aid on a broken leg! First you have to fix the ribs, then you have to re-caulk the seams. The 'wetness near the mast-step' needs further investigation. The region of the mast-step is the most highly-stressed in the whole hull, because when sailing the mast is pushing downwards on the keel, and trying to open the garboard seam. If you are not experienced at caulking, get someone who is to show you how, or to do it for you. It is very easy for a newcomer to get it wrong, and damage the plank edges.
Peter.