What is a type of thin flat cake made from cornmeal and salt water?

You're describing johnnycakes, also known as hoecakes or cornbread pancakes. These are flat, thin cakes traditionally made with cornmeal, salt, and water (though some recipes may include milk, sugar, or eggs). They are often cooked on a griddle or skillet and can be served with a variety of toppings, such as butter, syrup, or honey.