Help! CEL P0420

[kwkshft]

2009 Yaris S 23,0000 miles. No mods on engine, just a TRD exhaust which I've had for 3 years. Car is used for a lot of short in-town trips. I went on a longer trip last weekend. After the first 90 miles, I filled it since it was almost empty. Drove another 40 miles to Ann Arbor, MI. When leaving Ann Arbor, the CEL comes on. I immediately think it's the gas cap and keep going since the car is driving fine. I topped it up at the gas station that I filled it earlier in the day made sure the gas cap was on right. After a couple of days, CEL is still on so I take the negative terminal off to clear it. I made sure again that the gas cap was on right. CEL came back on after another 2 days. I took it to Advance Auto and their scanner said P0420 Bank 1. The guy there said its the O2 sensor. So did I get water in the tank? Thanks in advance.