David,
I would not hesitate to use marine plywood. I'm reasonably sure that was what the boat was built with. And, it's stronger. That said, you have choices. Are you planning to finish it bright? I recall Alarm's house sides were painted. If painted, any of the cheaper marine plywoods, except Fir, will be fine. Just spend an inordinate amount of time sealing the edges with epoxy. If bright, you could readily veneer the Fir or any other marine ply with Teak so that it will match the house sides. In fact, as I did on my new companionway slides, you could veneer the plywood in fairly thick stock, say 1/8" or more; if so you could use 1/2" ply. Tom Young has written several times about how to fix those cracks. A bit of time fitting dutchmen will take care of the rotted spots.
Our LeComte-built boat is all 1/2" Makore veneered plywood, including the bulkhead you're considering. However, the house back and cockpit is fiberglass. My conclusion is that the 1/2" stock is good enough for the lateral loads of a bulkhead but probably not good enough for the weather loads of the back of the house.
By the way, Delaware County Supply, in Boothwin PA, says they will bring in whatever marine plywood you wish; you needn't go far.
