This is a follow up to my previous question about mage hand: Does the Mage Hand cantrip pass through solid surfaces?
Last session, my Arcane Trickster was faced with some murky water and we needed to know how deep the bottom was. I decided to use mage hand to try to locate the bottom of the water without putting my own hand in there. We ruled that it wouldn't pass through the floor (which happily matches the conclusion of my previous question), but that raised the next question: do I know where it is?
The related question Do you need to be able to see the Mage Hand to use it? shows that you can move the mage hand without being able to see it, so not being able to see my mage hand once it had entered the water is not an issue (also, as an Arcane Trickster, it's invisible anyway).
Ordinarily, you know where your mage hand is because you explicitly move it, even if it's invisible, so if it starts in front of me as I cast it, and I move it 15 feet up, I know it is 15 feet above me because I moved it there; it can't be anywhere else, so I don't need to have any "locational feedback" from it in normal usage.
However, what about if the mage hand was not able to move to where I directed it to? If I moved it directly down by 30 ft, would I know (if, say, the water was 20 ft deep) that my mage hand only made it 20 ft and then stopped? Or would I be none the wiser to its exact location? Does the fact that I'm the caster make me somehow aware of its exact location, even if I can't see it, if something prevents it from moving as far as I directed it to?