In a nutshell, you get hypothermia when you body core temp drops below normal at a rate which your body's heat generation abilities cannot keep up. Mild Hypothermia takes place while the core temp is still above 90F. Profound Hypothermia takes place when the core temp drops below 90F. That is only about 9 degree F.
Any time a body is in an environment that is colder than 98.6 F, heat is lost. heat is lost to water conduction 240 times faster than to air. Heat loss in water in the low 70's exceeds the heat generation ability of the average person. Thin or small people, or people unacclimitized to the cold suffer the most. There is no significant difference between male and female bodies.
Not only that, primary heat generation is mainly done through exercise, specifically movement. This is actually detrimental in water due to the higher conductivity, as the movement pulls more cool water into contact with the skin increasing the cooling effect.
Hope that this helps