One interesting 21st century development has been the growing availability (and falling prices) of “recreational submarines.” Some military organizations, like U.S. SOCOM (Special Operations Command), keep an eye on all this because some of these civilian designs have potential military applications. One example is the Seabreacher line of submersibles. These craft weigh 658 kg (1,450 pounds), are 5.24 meters (17.2 feet) long, 1.06 meters (3.5 feet) wide, and seats two. Top speed underwater is 45 kilometers an hour and twice that on the surface. The 250 horsepower engine provides waterjet propulsion and very precise maneuverability. Seabreacher is not a true submarine, as it can only submerge as deep as its periscope mast allows (two meters/six feet max) because that mast (coming out of the dorsal fin) provides air and exhaust circulation for the engine and people on board. But that mast is difficult to spot from a distance and also carries video cameras and communications antenna (for cell phones or radio). What makes the Seabreacher really unique (as a consumer product) is its ability to quickly leap out of the water like a killer whale. Seabreacher costs from $70,000 to $100,000 (or more, depending on custom features).
For military purposes Seabreacher is ideal for scouting or landing small teams of scuba equipped SEALs (who can hang on to a Seabreacher equipped with external handles, as some already are for stunt work). Approaching slowly underwater, once the SEAL commandos have been delivered, the unencumbered Seabreacher can return to the mothership (or larger sub) at high speed. Seabreacher can also be used to scout a hostile shore, as any number of sensors can be mounted on the mast. Moreover, there is space in the Seabreacher for the new compact sonars to help it navigate shallow coastal waters or avoid naval mines. The manufacturer offers custom builds and if any Seabreachers have been built for military customers it is going to be kept secret.
Since the 1990s there have been a lot of recreational submarines. Luxury boat builders have even built submarine yachts. Submarine construction technology has come a long way in the past century, and it's possible to build these boats at an affordable ($10-200 million) cost. They are safe and there are over a hundred of them out there.
A few companies have gained a lot of experience building subs for non-military underwater operations (academic research, oil exploration), which has created a body of information and cadre of technicians who can build these recreational subs. One of the largest civilian submarine yards is in Dubai, where dozens have been built so far and construction continues. Another large operation in the U.S. has built most of the scientific subs over the last two decades.
The 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. 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. The 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 initially and later just employing the same tech used for recreational subs. Over a hundred submersibles (a sub that travels just below the surface) have been caught carrying cocaine. The age of privately owned subs is here.