1. Hair has two main layers. First an external thin layer called the cuticle that protects an inner shaft known as the cortex.
2. Hair is made up from protein known as keratin. Water can and does bind to this protein.
3. Hair in the cortex can gain and lose water.
So now on to the main feature........water is not a moisturiser at least not on its own. Take a moment to digest that statement, I am not saying water is useless, I am saying though that it may not be as useful in plain form. I have looked at multiple studies of moisturisation of skin and hair, it is clear that both will take up water but both will also lose water. The 'losing' part is the key to my previous statement. In fact water is sometimes used as a control (meaning if you are testing a moisturiser and want to compare its effect to no moisturiser, you spray the skin/hair with water and compare that to skin/hair with moisturiser). The question is what happens when you add water to hair? Well instead of writing a detailed narrative, I thought I might just make some drawings. In short, hair can take up water but it cannot hold on to the water if left in the same environment as before. Hair will lose any 'excess' water to balance itself with its environment.
Therefore if you simply spray hair with water and nothing more the additional water to the hair will eventually evaporate and you are back at square one.