Stringers, bulk heads, floors and transoms no longer only have wood as a option. Many composite materials are now available.
Composite boards like Thermo-lite, Core-lite, Kledgecell, Divinycell, Gruit and PVC foams will not rot and do not absorb water.
Also stocked is plascore honeycomb sheeting, polyurethane sheets.
All these products come in various thicknesses.