A corollary of Murphy's Law is that any buried conduit will fill with water. That said, I think that PVC jacketed non-outdoor network wire is just as waterproof as outdoor rated ethernet cable, and have a couple thousand feet of it in water-filled conduit. Some think I'm factually incorrect and/or crazy. Your decision, and YMMV. The only problem I've seen with non-outdoor cable outside is when it's exposed to sunlight. After a few years, the outer jacket can turn to dust. The outdoor rated cable is generally more resistant to physical damage from shovels, rodents, etc.