2002 Denali multiple cyl misfire

Put 10 gals of premium in yesterday, and within 1 mile of leaving station, engine started to bog. Got home, check engine light was flashing. Put hypertech reader on it today, and gave code of p0300, multiple engine misfire. Went back to station today and all midgrade and premium nozzles were bagged up. We did have ice storm a couple of days ago, but yesterday in the 40's.
I'm thinking water in fuel. Any help?

Welcome to AF. Maybe, maybe not, no way to guess at this, a fuel sample is required to test for contaminants, change the fuel filter and see what is in it.

Thanks. Will pull filter tommorow, and go from there.