How is submarine able to move on water surface as well as go under water ?

Fish are able to see under water because

Insects are able to run on the surface of water bacause :

The radius of the biggest metal coin of thickness t and density rho which would be able to float on water surface of surface of water is rho

Insect moves over surface of water because of a

Reptile living on water as well as on land is

The speed of a sub marine over sea surface is 30 knot per hour and below sea surface is 20 knots per hour.What is the average speed of the submarine if it is below water surface for 3/4 of her time of journey What is the average speed of the submarine if it is under water surface for 3/4 th of the total distance covered by him.

What is the radius of the biggest aluminium coin of thickness t and density rho , which will still be able to that on the water surface of surface tension S ?