Written driving instructions at the bottom of the page:

Driving Instructions from Christchurch to Lake Clearwater and Camp (Refer to Above Map)
(Via Thompson's Track)
To get there take the Main South road out of Christchurch toward Timaru and turn a hard right at Rakaia township (Immediately after the huge fibreglass salmon on the right hand side of the road). Follow the signs to Mount Somers and they turn into signs to Lake Clearwater.

Lake Clearwater is a small lake, it is a wind-powered-craft-only lake since it is a nesting area for the Southern black crested Grebe (population 250, according to the sign). Nesting areas are well sign-posted and fenced off in some areas, and it's important to have a quick look at the Forest and Bird sign to see where you're allowed, and, not allowed to skate.
For power boat heads Lake Camp is 5 minutes walk, which does allow powered craft.