DEFINITELY a one way ticket. You would eventually use up all the oxygen in the bell unless there was a vent to bleed off some of the air to get rid of the CO2. Staying down too long would deplete your air supply causing you to stay down forever …
Some local fellows made one to salvage some whiskey from a sunken packet boat after the war. They hand pumped air to a hard hat type device. It worked pretty well until the diver bent over and lost his bubble. He had some trouble surfacing with the lead weights, but did. The whiskey was found to have been in damaged barrels, not intact jugs.