The periscope, basically a vertical tube with mirrors inside it.
A Periscope or Radar
periscope
Normal ships are designed to move on the surface of the water. Submarines are designed to be able to submerge and travel under the water's surface, as well as on the surface. When normal ships go down, they stay down. Submarines are able to come back up after they go down. All American submarines are nuclear powered. With the exception of Aircraft carriers, surface ships are powered conventially.
Submarines cannot reach the deepest parts of water because of the pressure or water and deepest surface is not a smooth, its full of rocks and sea plants.
Submarines have ballast tanks inside them that can be filled with water to make them sink.
Periscope
Under water bombs are called naval mines. Under water bombs are used to destroy surface ships or submarines.
Submarines dive by opening vent valves that allow sea water to push air out of their ballast tanks.
Submarines have ballast tanks in them.........when it has to go deep down, the ballast tanks are filled with water, which increases its average density in regard to water(Relative Density).....and when it has to come up to the surface it empties the ballast tanks...and it rises. That's it.
Submarines change their depth by use of ballast tanks. To dive, they open their tanks so that they become filled with water. They also angle their dive planes. To surface, they pump the water out of their tanks and fill them with pressurised air.
a persicope
It depends on the submarine type and hull design. Older WWI and WWII Fleet submarines were specifically designed as surface ships with limited submergence capability - they primarily ran on the surface with their diesel engines, and submerged using their battery powered electric motors when attacking a surface vessel. This hull design made gave them a lower water resistance on the surface, but much higher underwater. Their speed underwater was almost half of what they could do on the surface. With modern submarines, the opposite is true. Submarines today are specifically designed for lower water resistance (higher speeds) underwater than they are on the surface. Top speed for most boats on the surface might approach 20 knots, but underwater they can do over 30 knots. The old Russian ALFA class submarines were known to do 45 knots submerged.