Semi-submarines Vs Submarines 
There are significant differences between a submarine and a semi-submarine from an engineering point of view.
Submarines that we refer to are human occupied pressure vessels. Failure of a submarine during operation induces fatal injuries or mortalities. Therefore very effective rules have been established to ensure safety at any time. The certification process of a submarine is very strict and results in tight limitations to the design of submarines.
All this applies to a lesser extend to semi-submarines. As the pressure close to the water surface is relatively low the viewports can be bigger. Customers can leave the submerged cabin to the unnumbered deck at any time.
Some very great designs have been developed by the French architect Jacques Rougerie. In some advanced designs the windows enclose the majority of the submersed hull. A great experience - only compared to scuba diving can be obtained.
