I definitely have to agree with using something non absorbent and it definitely wouldn't hurt to raise them about a 1/2" or so. Though I'm going to contradict myself a bit, I have two safes, one that was bolted directly to a tile floor and one, that weighs 3400 lbs, that had to be elevated on two 1/2" x 4" strips of hardwood because of the jacks used for moving.
Back in 2008 the building where I kept the safes was flooded with about an inch of water. After several days I "thought" I had everything dried out. I even moved the smaller safe and removed water that was underneath. Moving the larger safe wasn't going to happen without professional help so it just stayed in place. Recently, I moved to another house and had the safes moved. When the larger safe was moved, I was able to peek under finding no rust despite having the hardwood strips. The smaller safe, though I originally moved it and dried underneath did have light rust. Its evident that the small strips of hardwood didn't retain mosture.
Both my safes in the new location are now elevated 1/2" just in case.