If you are talking about Nidacore, I had a friend use it, and I helped a bit with the tabbing on the deck. I would use no other product. Fully waterproof synthetic (plastic) core, you glass both sides with perhaps 1708 with the number of laminations dependent on the maximum span between spar centers. Below is a post that I collected from a professional boat builder that says it all. Remember that almost all foams will absorb water over time, better than wood, but not the best available, and a reasonable price similarity.
Joe
Post
We started using Nida-Core about 10 years ago for saloon decks (3" clear span), then added forepeak (1-1/2"), cockpit (1-1/2") and now foredecks (3/4") too, all the decks in the boat our 60+'ers. I guess you can say we like it! Quiet, easy to work with, excellent peel strength and price isn't linear with thickness like the foams. Depending on your spans you can probably go with 1 or 1-1/2" and yes, you do have to glass both sides. Our "standard" layup is two layers of 1700 cloth each side and you can span 24-26" with 1" and 30-32" with 1-1/2" with that skin thickness. Comparing like thickness 3/4" it is nearly the same price as a good grade marine plywood and 1/3 the weight when finished!