IMO, it is not possible for water to seep all the way through the spark plug theads without it also being possible for exhaust/combustion byproducts to escape.
Do you hear the sound of escaping gases on the driver's side? Did you see evidence of combustion byproducts on the old spark plugs?
If you have water standing in the spark plug wells, it can cause a misfire by grounding out the ignition. Further, if allowed to continue, this will damage the boot such that even when dry, the misfire will still occur. Note, for COP applications it is possible to buy just the boot. Much cheaper than than the entire COP.
I am presuming that since the plugs were changed, there is no more water in the wells. Further, I assumed that the problem continued after the spark plugs were changed.
If there truely is water on the all of the driver's side plugs (from the inside), it has to be getting into the intake via the intake gasket or some other defect. If true, I would expect the car to run like total CRAP. The vacuum leak would have to be huge.
Since you seam to think this is an O2 problem, change it out as generally around 100k miles, the O2 sensors do tend to have slower response time.
The best way to check for vacuum leaks is with a smoke tester. However, this is out of the price range for most home mechanices.
The next best is a propane based leak detector.
Then comes good old visual.
Some ppl may suggest spraying carb and choke cleaner (or brake clean). IMO this is a bad idea because of the very real possibility of gasket damage. This will have you trading one problem for another.