But it's ultimately proven better-suited to the pens of fanciful designers than the realities of the marina. With the Nautilus, U-Boat Worx seems determined to bring the concept to reality.

It puts its considerable experience in personal submarine design into the impressive 123-ft (37.5-m) vessel, which combines the usual trappings of superyacht life – sundeck, pool, bar and more – with the unique abilities to dive down to 650-ft (~200-m) depths and stay underwater for days at a time.  We always imagined the personal submarine market to be a rather leisurely one, filled with extended testing and development periods submerged in glassy-blue tropical waters. But U-Boat Worx has been absolutely grinding lately.