Thanks for all the input. It's nice to see all the different methods everyone used.
I'm really sold on both Vinylester resin and Corecell. Vinylester works just like polyester fast and easy. When I researched Corecell I saw that it is used considerably in the megayacht market. Yes, it's more expensive that the other coring materials out there, but considering what a recore job would cost in a shop, the cost is minor. Also, it's touted to have some better properties than the other foams. Can't say any more than it worked out fantastically on the 100 sqft job I did with it. Six years of heavy use and an impact prone cockpit sole fully recored with Corecell showed no problems (as well as all decks 100% dry) when I sold the boat. It really does make a superb light structure. Sorry if I sound like a salesman
One issue I've seen over and over in boats up north in the great lakes especially with plywood coring is the continuous freeze thaw cycles up here. Literally hundreds of freeze/thaws a winter. Once water enters the structure the freezing expands the water. The thaw allows more water to fill the new void created. The tremendous pressure that freezing creates rips plywood coring to pieces. Balsa goes to mush. I was looking for a core that is 100% water repellant. The closest I found was the Corecell. Keeping a deck completely free of water intrusion is a primary concern up here.
An advantage of all the foam corings is they are easily shaped after they are bonded down. A 40 grit disc on an angle grinder makes short work of blending in odd shapes and curves. I ran into a lot of thickness issues when the coring was laid up and I can't imagine the work it would have taken had I used plywood. Also foam cuts easily with a utility knife. Just score deeply and snap it. I had hundreds of small jig saw puzzle pieces cut to do the deck of the Hunter.
After doing that job I will admit that plywood and balsa would serve as well as any other cores. Every core failure I've seen was caused by shoddy manufacturer builds. I made absolutely sure all coring joints were 100% filled and sealed with resin. Even the best boat builders could pay far more attention to core lay up and sealing (that's unseen expense though and doesn't sell boats).
I like the idea of vacuum bagging. I have an old refrigeration vacuum pump and it sounds like vacuum bonding would be a great way to go. I could possibly do a dry vacuum fit to see if everything looks good before applying resin. Interesting....
I'll post some photos as I proceed. I have the swim platform gutted and ready to start a recore and rebuild of that. The plywood core literally poured water on my shop floor. ....but then, it's 40 years old. Maybe plywood ain't so bad???