A person with dark hair in a white shirt is sitting in a chair in water with a 'swim at your own risk' sign on the wall.
a person sits in a chair in water
A person is described in the first sentence as sitting in a chair in water while the second sentence explains a person sitting in a chair in water.
Person with dark hair is a type of person, and sitting in a chair is a paraphrase of sits in a chair.
Sentence 2 simplifies the first sentence to say that a person is sitting in a chair in water.