Basically the strength is from your lay-up. There is a formula - when the skins and foam are added together the thicker the foam the stronger and stiffer the laminate as compared to a straight laminate. Some say you cannot do better then wood. Foam cost more but it is lighter and does not rot. Do you have a picture of your project perhaps a before is it a Wharram Tiki? I found a builders / Wharram website http://www.wharram.eu/cgi-bin/yabb2/YaBB.pl
There are things to plan for with foam such as masts, fittings, hardware, Crossbeams, ect...
From what I've seen most of the Tiki boat are built on frames and bulkheads so foam should not be a problem. One other thought is Balsa coring it makes a stronger and stiffer deck but it is subject to rot.
To answer your question yes I'd go with the 1/2 foam as your core thickness would be the same.