1999 Miata - Check Engine - Help

I need a few words of wisdom about my '99.   It has 65K miles on it.  About

a week ago I did something that I believe was very stupid.   I cleaned my 
engine by hosing it down.  The car ran ok for a little while, but then 
stopped.  The engine light was on. Yes - you guessed it.  Water in the
plug 
wells.  Ugh!  I spend the time to remove the water and dry the wires &
plug 
wells.  The car was running fine for another few days afterwards and then 
the check engine light returned.  The autozone tech told me the trouble 
shooting error code is P0420., which is:  "Catalyst efficiency low-bank 
..  - the oxygen sensors monitor the catalytic converters ability to store

oxygen.
Possible causes are:
1. Catalytic converter defective (Failure possibly due to #2, 3, or 4);
2. Engine msfire or running condition;
3. Large vacuum leak;
4.  Engine oil leakage into exhaust-value guide seals, piston rings

Any suggestions on approach to uderstanding and fixing the problem?

I've been considering replacing the plugs & wires.  Is this the correct 
first step?

Thanks,
- Pat