Marine Grade Plywood?
I appreciate the advice. $220 a sheet is WAY beyond my budget.
I''m thinking that I shall be making a smaller (20-25) ft daysailer which will be taken out of the water and stored in shelter after each use... and that I''m going to have to be practical and go with normal, exterior grade plywood. I shall have to look into the details of foam core construction as my knowledge level is too low to be useful on that one.