If there's a downside to having a submersible as a superyacht amenity, it's that most of these vessels accommodate only two or three, maybe four, occupants, while yachts large enough to hold a sub can host two or three times that many passengers. Consequently, many guests may have to wait their turns to take the plunge.

The new Triton 1000/7 ( triton​subs.com) from the Vero Beach, Fla., company Triton Submarines allows for a more communal underwater experience.

As indicated by the model number, it seats seven occupants and can reach a depth of 1,000 feet (305 metres). It's designed to be operated from cruise ships as well as superyachts with sufficiently spacious tender garages.

The 1000/7 is 3.8 metres long and 2.5 metres tall. Its battery lasts for 18 hours when fully charged and powers thrusters that enable a speed of 3.5 knots. The price is $US4.9 million (about $A6.5 million), and delivery time is 2 years.