Lake Clearwater (Wikipedia) is a small South Island lake which can be found some 34km north west of State Highway 72 with the turn off near Mt Somers. Half the road is sealed although the unsealed section was of pretty decent quality the day I drove in even though from the water lying about there had been some serious rain in the not to distant past.
There are actually a series of lakes along the road - I couldn't find the name of the one above which is about half way along the unsealed sealed section and on the northern side.
Expect to see a lot more sheep than people and come prepared as there are no shops, toilets or facilities of any kind at the Lake which is mainly popular with fishermen and die-hard windsurfer types due to its wind-swept reputation. These days there is quite a settlement at the lake with two "streets" of these modest holiday homes. These are pretty much "off the grid", solar power, gas appliances, log fires and definitely no cellular service - sounds ideal