Why is it that a ship does not sink in the ocean, as heavy as it is?

Ships do not sink in ocean and actually float on the water surface, simply because of buoyancy. Liquids exert an upward force on bodies immersed in them and this upward force is known as the buoyant force. According to the Archimedes Principle, the amount of buoyant force is equal to the weight of the liquid displaced by the immersed object. 

Thus, to float on the water surface, all a ship has to do is to displace a higher weight of water than its own weight. This way, a higher buoyancy force (in upwards direction) will counterbalance the ship's weight (in downwards direction). This is done by increasing the surface area (which is in contact with water) of the ship, distributing its weight and keeping large sections hollow. This causes more water to be displaced and keeps the ship afloat. Smaller ships and boats are made up of wood, instead of metal, to keep the weight low and to ensure that they float.

Hope this helps.