I dont know a lot about boat building, however I know a little about ply, there are many types of ply, the standard ply in the builders merchant is not a good quality. The marine plys have more laminates of a better quality timber and water resistant glue is used, the price is also much more expensive and the board is stronger and slightly heavier. The capping with resin and glass fibre should waterproof the sandwhich and when drillng holes fill them with sixafix or similar to keep the water out. I believe modern boat builders are moving away from this method but it was good enough for many years. By the way is the channel shown where the ply locates, if so was the old ply sucking up water held in the channel? I assume also the transom would be at least two thicknesses of 18mm ply.