An SPG (Submerged Power Generator) is fully submerged underwater like a submarine. When you are inside an SPG you are not in water. The air inside an SPG is similar to being inside a boat cabin or walking on land. You have to understand this first before you can understand that the electricity an SPG makes is made exactly like a generator next to a dam.
Click the link below for a very small movie to view.
How Does a Submerged Power Generator -SPG- Work
How is an SPG different from a dam? One important difference is how the falling water spins the turbine to drive the generator and another difference is how the water flows.
An SPG puts water inside a container that is very large! The large container is engineered to have a shape like a torpedo that is fired from a submarine for 2 reasons;
1) The shape of a torpedo is engineered to move through water quickly and efficiently &
2) A torpedo is engineered to be loaded into an airlock (called a “torpedo tube”) that allows the torpedo to be safely ejected into the water without sinking the submarine.