A LOT of times deer go down to water to die, she could very well be hidden down in some reeds or thick underbrush hidden from view.
I have even heard of a deer shot back too far getting in the pond and died. They saw a little bit of him floating and some how managed to retrieve him. I am told a deer will "feel the burn" and head for the water to cool it off.
9 years ago my buddy shot an 18 pointer that died in the trout stream. Didn't get found for several weeks. Chest cavity was well preserved by the cold water and showed a very good looking hit.