Over the last two decades luxury boat builders have begun building recreational submarines in large quantities. There are now hundreds of them in use, some built by hobbyists. All this is the result of decades of new developments in materials and engineering techniques that make constructing effective subs cheaper and easier.
At the low end you have hobbyist home-builts that cost less than $50,000 for components, plus over a thousand hours of labor by the do-it-yourself submarine builders. Commercial subs start at a $100,000 of so for small one, two, and three seaters. Some of these low cost subs can go down 1,000 meters (3,100 feet) and are sometimes used by ocean researchers. So far no one has been killed using recreational subs, and over ten million people have traveled underwater (usually short distances as part of a tourist activity) in them. There is also a growing second-hand market.
Submarine construction technology has come a long way in the past century and it's possible to build these boats at an affordable cost, although some of the luxury models cost over $100 million. They are safe and there are a growing number of them out there. This is driven, in par,t by the demand by ocean researchers, oil companies, and other underwater commercial operators who need small, affordable submarines. A few companies have gained a lot of experience building subs for non-military underwater operations, which has created a cadre of information and technicians who can build these recreational subs.
Some of these submersible pleasure craft look like streamlined yachts while on the surface. The upper deck, including the bridge, is outside the pressure hull. When submerging everyone goes below and the upper deck gets flooded. If you get close to one of these yachts it becomes obvious that they are built to dive. Military subs are still not used to encountering this civilian traffic underwater. The military boats have the right of way but military boats are now warned to exercise extra care when approaching coastal areas used by civilian subs.
Owners of these luxury subs tend to be secretive and the builders have agreed to some government oversight, especially to make sure militarized subs, that can carry torpedoes or mines, are not built using the civilian technology. But there is no law against anyone owning one of these submarines and it's feared that it's only a matter of time before drug dealers, gun runners, or even terrorists get their hands on some of them. Some police officials believe this has already happened but no one is saying much. In South America there is already an illegal submarine construction industry turning out boats for transporting cocaine.
Most civilian subs don't dive as deep as military subs and are not built for combat. They have staterooms and large windows. But they do have carrying capacity and that could be put to criminal uses. Already Colombian gangs have been caught trying to build subs, using Russian advisors. And at least one submersible (a sub that travels just below the surface) was caught carrying cocaine.