Are you talking bulkhead or floor?
If it's the bulkhead, roted wood cannot be restored. The bulkhead is birch plywood covered with formica both sides. It roted because water reached the base at the deck - it was probably not caulked properly at the factory.
You can remove the section and take it to a fabricator to use as a template either for 3/4 startboard or whatever you want.
If it's plywood, the end grain needs to be sealed with epoxy, then caulk the base well. Any holes or screws into the bulkehed need to be caulked also. Follow this plan, and it will never rot again.