both normal and direct bury cabling have the same outer PVC jacket.
direct bury cable differs in that there is an additional internal filler -- a gel-like substance that is intended to help prevent corrosion of a metallic conductor in the event that the outer PVC jacket is damaged (e.g. a small nick or stretch/radius fracture) during installation of the cable below ground or in a conduit.
you are correct that any underground conduit will eventually fill with water due to condensation. you can use NASA-grade solvent welds on your underground NM (PVC) conduit joints but the condensation due to the lower soil temperature will eventually fill the conduit with water unless the ends of the conduit are hermetically sealed (which is all but impossible). for this reason, the NEC rates underground conduit as a "wet location" and requires specific types of insulation (e.g. THWN) for AC wiring.
multi-mode and single-mode fiber optic cables are both marginally affected by water absorption in the plastic fiber (MM) and/or plastic cladding (SM), however at the distances most residential folks are going to encounter (< 2Km) the added attenuation due to water absorption will not be a significant factor in the optical link budget.
wrooster
