I know its expensive and a bit (perhaps very much) against the design philosophy, but what about using Airex or even honeycomb and glassing that both sides? Should come out lighter from about 6mm up IIRC. What's the thickness of the decking you have?
OTOH, if its all too prone to rot, perhaps the builder used substandard quality? ie non marine ply? Replacing that with proper material (drenched in epoxy, perhaps even sheathed) should help.