Water in my trunk

While cleaning my car, I discovered that I've been getting water in my trunk. The source is unclear but it might be that I'm getting it collecting through some hole near the wheel well. The jack has a crevice of its own and appears to have been soaking in water for at least a month. The rust has destroyed it as a jack. The screw holding it in was rusted and it took me 20 minutes to get it undone, using tools I'd not have had in the car if I had an actual emergency. I don't know how to find the leak (so can't plug it). Perhaps some sort of overall spray sealant?