Do fire extinguishers require replacing if they were in a flooded building where the water was 3 feet up the wall and would have been in the water or at least the nozzle was.

Maybe, maybe not!! Co2 - effectively a pressure vessel - flooding as such shouldn't cause degradation/leaks (corrosion is a long term issue). Clean down, instructions still readable? Discharge pipe not damaged/missing? Locking/indicator pin still in place? Should be ok, I would think. Dry Powder - maybe/maybe not - if just the lower part of the body is wet - then treat as above. I would think the only concern is if flood water can enter the body of the extinguisher via the top head screw connection and hence contaminate the powder inside. Other than that same as check labels etc Water - Similar argument, provided the head hasn't become loose, can't see flood water entering the body of the extinguisher. Clean down externally, labels/locking devices and missing discharge pipes? etc. The gas cartridge inside should be ok. Do you really need to be a 'qualified' fire extinguisher 'engineer'?