How To Build A Sub Marine That Can Float, Sink, And Float Again?

Well, make a submarine large enough to have a container for water storage, get a comparative size water suction mechanisms (also found in fish aquariums), and put all the water inside water container. Make submarine from a floating material, or use upward thrust of water i.e. Make such a design that float in initial resting position along with water container and water suction mechanism. As the weight increases, the submarine will start sinking in water.
Once it is in deep water, the way to bring it bak to floating point is release of inside water in the container to surrounding environment again with the help of suction mechanisms.
Now, supplying power to these mechanisms and the control circuit which will control the sinking and floating, can be done by computer interface or manual control, or control circuit using some control devices like micro-controllers, PID or PLC controllers.
In case of wireless control or automatic timing control of sinking or floating, the entire control assembly along with some water proof batteries are required.