Can a circuit breaker become compromised by flooding?
Watch for flood lines.  Yes, any breaker that's been underwater should be smashed (so others don't dig it out of the trash and use it).  

If you can't find an uncorrupted main breaker to cut, call the power company at the "my power is out" number and they'll send someone over ASAP because they don't want you dead. Dead customers don't buy electricity.   However if this was a large scale incident, their plate will be very, very full and your request will get triage'd and done when feasible.   Obviously, in the meantime, stay away from it.   

After the flood is gone you have to size up each component.  Panels are simple affairs and it might be possible to take it apart and clean it.  Conduit should be OK but I'd pull the wires out and disassemble it enough to clean the insides by pulling a rag through them.  Every junction box would have to be cleaned out.   The wires will be ok if they say THWN or other outdoor rating.   Ratings for outdoor wire presume the wire is immersed in water 24x7.   Keeping water out of outdoor conduit isn't really possible.  

Romex will be junk.  Every receptacle, switch and machine of any kind will be junk.