My C10 is starting to have the same problem. It's cause any water/snow dragged in from boots pools in that lip formed where the rocker and floor meet, it's pretty common. I'm going to chop out the rocker and just buy a 1/2 floor patch, and make my own kick panel replacement.
Just have at 'er, if you can't weld very well then it's good practice and if it looks ugly, it's gonna be covered up by the carpet anyway.