Adding water would have an effect on your product's pH but probably not enough in small quantity and you'd end up drowning your product. To raise the pH of your homemade shampoo, you can try dissolving 1g of baking soda in 10mL of water. Then, add drop by drop into your shampoo and mix in-between until the pH is up to 4.5 (like for lowering the pH with lemon but the other way). But do keep in mind that if you have to add too much of baking soda in your product, it will be pH balanced but the overall result will be harsher so this is only a last resort option :)