Diving Bells are apparatus designed to extend hundreds of miles down to planetary bodies, and withstand the immense pressures under the surfaces of ocean moons.
The first one used was not seen, but it took the gang down to an ocean moon where they discovered the submerged wreck of the SSS Esperanto. Although the Diving Bell is not seen, the gang are seen weeping uncontrollably in the Starbug re-pressurization room as they are affected by the ink of the Despair Squid. ("Back to Reality", Series V)
When the original crew of Red Dwarf were resurrected by the nanobots, the regular characters find themselves locked up in the ship's brig by Captain Hollister ("Back in the Red"), and soon join the Canaries, a volunteer prisoner "suicide squad". Their first mission is to investigate the submerged wreck of SSS Silverberg which Red Dwarf had come across, which the Diving Bell takes them down to from the Canary shuttle. ("Cassandra", Series VIII)
Many years later, the gang once again had a full run of the ship to themselves, but find themselves having problems with their own Water Tank on Red Dwarf, and take a Diving Bell down into it to investigate... (Back to Earth)