A system for use in a subsea well includes a sealing element having an inner surface defining a bore through which a carrier line of a tool string may extend. A pressure-activated operator is coupled to the sealing element and is adapted to cause the sealing element to deform generally radially inwardly to allow the inner surface to apply a force to seal around the carrier line. A fluid pressure conduit extends from a sea surface pressure source to the pressure-activated operator. The sealing element is part of a pack-off device that can be used in a subsea blow-out preventer.