Queenstown has this little place called the Underwater Observatory. It's a little building that stands on the bottom of the lake. You can see directly into the lake, see the fish swimming around, the eel curling around the food baskets and the ducks diving from up above to fetch a little treat for themselves as well. I have never seen water so clear. None of the pictures below are edited, it's exactly how it looks down below.
Fun Fact: The water in Lake Wakatipu is so clean that if you're in the middle of the lake you can actually safely drink the water from it.