The water intrusion probably started above the actual bulkhead. So make sure that you seal the new work from the problems that are above where you are working. My personal opinion is that the marine plywood would be adequate, if it is well sealed with epoxy resin. I have removed a lot of the skin below the waterline and would not even bother with considering to reuse it. Once it is separated from the core it often wants to take on a new shape that just doesn't fit back in the place it came from. That skin is also polyester resin and chopped strand mat, not particularly strong, so the best place for it is in your garbage can. It is also just simpler and faster to reglass any of these areas.
I also highly recommend using a high volume vacuum when you do the new glass work. This can even be a shop vac. Using the vacuum helps with fighting gravity and really helps consolidate the layup. With the high volume setup you can use blue painter tape to seal your plastic to the hull and a simple sink drain as the vacuum port. The low volume high vacuum pumps also work but require much more substantial sealing effort and expensive materials. Here is what this can look like
Vacuum Port
