Catalytic converter

I have a 2001 SLK 320 with 26K miles. My mechanic tells me that I need new left side catalytic converters, because they are beginning to rattle. Replacement cost is $1300 plus labor. My car was water damaged during the 2001 flood in Houston, TX. No water was ingested into the engine, and water did not enter the cockpit. However, some water may have entered the tail pipe. Assuming water may have reached the catalytic converters (don't know if this is physically possible), would water cause the converters to fail? Answer to this question is important, because Mercedes has chosen to review case-by-case warranty claims on my car. I want to be prepared in the event they refuse to warranty the catalytic converters? Also, does anyone know how much discretion the mechanic has in making this determination? Thanks--