There are many instances when you may want to pass electrical wires (such as a tether, motor wires, communication lines between compartments, etc) through a waterproof barrier. A cheap, easy, and effective way to do this is with a potted pass-through. The basic idea of a potted pass through is that an initially fluid material (such as epoxy or potting compound) can be poured around electrical wires that are going through a tube. When the material hardens, it forms a wall that water can't penetrate. By attaching the potted tube to a second (dry) tube using vinyl hose material, the connection can be removed easily without having to destroy any parts or cut any wires. This diagram and photo should give you a better idea of how everything goes together. Note that in order to keep the pass-through from leaking, you should sand and clean the outside of the wires and inside of the tube so that the potting material bonds well with both. Many compounds will work well, but my personal favorite (due to having just the right amount of softness) is Loctite Hysol E-90 FL.
Trick 1, Figure 1: Diagram of Potted Pass-Through
Trick 1, Figure 2: Potted Pass-Through being implemented on ROV1 for motor power wires.