A family holiday home that was effectively a submarine

What is the weirdest boat or ship idea you have encountered?

A self-propelled underwater family holiday home that its designers resisted calling a submarine.

The proposal was for a relocatable, self-propelled underwater living space that families could hire much like a camper van. Ordinary family groups would crew the craft during Caribbean holidays. Its designers resisted calling it a submarine, despite the vessel's intended operation. Changing the name did not remove the difficulties of having holidaymakers operate a submerged vessel. The account deliberately alters some of the project's original terminology and does not identify the company. It also distinguishes these designers from the seasteaders suggested in the question.