Semi-submarine
Semi-submarines are surface vessels that are not capable of diving, with accommodation space below the waterline featuring underwater windows, like glass-bottom boats, but deeper. They are mainly used to provide sight-seeing trips for tourists in clear, calm, and often shallow, waters.

Design 
The most common design is similar to a ship. The passenger cabin is deep in the hull, a few meters below the waterline. The cabin is equipped with large underwater windows so the passengers can observe the marine environment that is passed during the voyage.

There are significant engineering differences between a submarine and a semi-submarine. Submarines are human-occupied pressure vessels exposed to high external pressure, while semi-submarines are exposed to the same pressures as other surface vessels of similar draught operating in similar conditions. As the hydrostatic pressure close to the water surface is relatively low, the viewing windows can be large. In some designs the windows enclose the majority of the immersed hull. Passengers can leave the submerged cabin for the unsubmerged deck at any time.

Use 
Semi-submarines can be used for research, but they are most commonly used in the tourism business. However, large tourism-oriented semi submarines should not be confused with narco-submarines which are smaller home-made semi-submarines used to smuggle drugs.

Legal status 
Semi-submarines do not have an international classified status. Their operating range from the native port might be limited by the local authorities.

See also 

Semi-submersible naval vessel

External link 
Turkey's first semi-submarine

Boat types
Nautical terminology
Water transport
Tourist activities
Submarines
Pressure vessels