Actually, I've been to Marleybone and I understand what the balloon cars are like.
This is my picture of an actual balloon car mount:
On land, it would be all lit up and up in the air. The owner is in the driver's cabin while the other passengers are on the rim looking out at the view. Underwater, however, the balloon would be deflated and all of the passengers would either be cramming in the driver's cabin or attached to harnesses because you could easily float away. Plus, it would make for the best water experience.