I recently learned that agua is a feminine noun, despite the fact that it is used with el and un. This is because la agua "sounds strange" with the a sounds coming together. Apparently this also applies to other similar nouns. I am curious why there isn't something similar for mi hija (or mi hijo). Would the same principle not apply?
How would it be changed? With 'agua' you have the option of one of two different genders. With the phrase 'mi hija' you just have the word 'mi'. It doesn't change into a different word that nonetheless means the same thing, as you have with 'el' and 'la'. The only variation (that I am aware of; I am by no means even semi-fluent) is 'mis', which is merely plural.
It is certainly pronounced that way. I have seen it written that way at least a few times. That might not be the standard in some grammar books, but... this is one of those cases where I might say "non-standard" instead of just "wrong." I'm keeping in mind that standards change. Words are added to dictionaries, some are removed, and so are meanings (added and removed).