At first I thought "Why not connect the sub using a cable to a ship on the surface for communication?" Then I thought "Oh wait, it's 2.5 miles down, no ship could carry that much cable." Then I read about an unmanned sub on the ship L'Atalante: It has grappling arms that could have retrieved the Titan (had it been intact), and it's connected to the mother ship by FIVE MILES of cable.