How do I know this? Unfortunately... personal experience. My daily driver is a 1967 Volkswagen Beetle. Driving home from work, I ran into some flash flooding. Needless to say, I wound up drifting across an intersection in about a foot and a half of water. I was pretty lucky that there was no major damage done. I did suffer from a squeaky starter bushing for a while; easily fixable with a dab of wheel bearing grease.
I did however have about 3 inches of water on my new floor pans. When I welded them in last summer, I sealed them with 3M Fast-N-Firm seam sealer. I now realize that was a mistake. The stuff gets real brittle and doesn't seal worth a darn. I chipped away the old sealant and used TigerSeal. This is some really nice stuff. It goes on easy, is polyurethane based, and is very flexible. I'll definitely be using more of this stuff in the future
