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There may be a simpler method that I have been teasing to do myself. I had my hull surveyed and water intrusion (measured with the moisture meter) that was located by my drain tubes. My drain tubes have been out for a season air drying the area (I have been sidetracked working on a commercial salmon boat). As stated I have used the calcium chloride in the past. Sealing the area is the key. With my drain tubes out I have air access to the foam. Where convenient, this winter I may plug the hull side of the through drain and enclose the area on the deck side/cabin side (I have a 1979 22' Revenge) with plastic and a tray of calcium chloride and see if I get any type of moisture collection. This it the method used when measuring the moisture in concrete (you need a moisture reading in concrete for some staining and overlayments). I am pretty certain there will be a water collection. Before I start I will get the boat surveyer out here again to see how well the foam did with the drain tubes out. Calcium Chloride is the active ingredient in most moisture collecting products. You do have to seal it off from atmospheric intrusion to work. If a seal was broken with my concrete testing (I did several on the same slab) the results are thrown off. If moisture can be drawn from hardened concrete I do not see how foam with air access would be of any hindrance.
Garris