Hello, Pelin. As a native English speaker, but not a qualified teacher, this is my answer: Is the water shallow/deep? Is it (the water) shallow/deep? We say this when we are speaking generally about the depth of the water in the lake. Is the water shallow /deep there? We say this when we are speaking more specifically about the depth of the water in the (exact) place where you are standing. In my opinion you should really only use the word " it" when it is clear to both people from the context what 'it' means, otherwise this could cause confusion/misunderstanding. In this context the word it clearly means/is referring to the depth of the water. I hope this helps you 😀