Question

why does a ball made of steel sinks in water while a ship made of steel floats?

Answer 1

Because of
Law of flotation.

A floating object displaces a weight of fluid equal to its own weight. Every ship, submarine, and dirigible must be designed to displace a weight of fluid at least equal to its own weight. 

Answer 2

$\bf\huge\underline\mathbb\green{ANSWER}$

The ship displaces more water than its own weight whereas the weight of the water displaced by a steel ball is much less than the weight of the steel ball.