one of the benefits of 100/130 avgas is that is does not mix with water.
I once took over half an hour to drain all the rainwater out of a Cessna 150's tanks. I took a bit of rocking to persuade the final bits of water into the sump but once done the engine ran just fine. we replaced the caps after that.
you will only ever have problems with fuel and water contamination if you are silly enough to use a fuel with alcohol in it.