Construction below flood levels, within regulated flood zones, must utilize flood resistant materials.
In this environment, the use of untreated wood, paper-back gypsum or non-galvanized metal below the flood level - or in contact with the foundation - will quickly lead to rot, delamination and degradation.
In the FEMA publication "Technical Bulletin 2 (TB 2)," there’s a handy reference chart to quickly determine which materials are considered acceptable and those that aren’t. Start with Table 1 on page 4 Class Descriptions of Materials. Then compare the different classes with the list of materials on Table 2, on pages 7 to 11.
Additionally, this publication has a wealth of related materials and methods information.