How well do whitewater kayaks work for paddling around a lake?
Not Very Well…
They don’t track well because of the rocker. You have to have good technique to paddle the boat straight. Doesn’t have any speed. All can go to heck in any significant wind of 15 knots or more.
Other than that, practicing white water moves on a lake is a good way to kill a summer evening. 
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given enough time and paid hourly
you can do anything in a whitewater boat
How big of a lake? Whitewater boats are slow, some older longer whitewater boats are not too bad for long distance paddles. I do short trips two or three miles on the coast in my Necky Jive to work out and get some excercise once in a while.
More detail (please)
You might get better information if you provided more details as to why you are asking this. If you have a particular WW boat in mind, then you should mention it.
Anyway, for short trips a WW boat like an RPM is OK to paddle around a small lake in.
If your goal is to become a stronger WW paddler, then any WW boat would be OK.
Some good, mostly bad.
Most, especially newer designs, are quite stable and all are very maneuverable. Beyond that you have to be in it for the exercise or improving your boat handling skills or flatwater play moves. I have a friend who used to paddle around the lake I live on in his Wave Sport EZ at “top speed”. It is a moderate sized lake and he would go around 2 or 3 times. Needless to say he was in good shape but, I think, a little masochistic.
I would do it if it was a really
really small lake and if I needed that particular exercize. I only go to a lake in my ww kayak to practice rolls, I would otherwize find it a royal pain for the reasons others have noted. A quick answer is no…
waves!
A few weeks ago saw guy having lots of fun in WW boat on lake Erie. He wasn’t going to far in though.
good way to meet chicks.
Ya pull into the lakeside bar with your ww yak.Yah I’m bad I’m bad
On our loacl lake sometimes you can…
…surf the boat wakes in a WW boat.
But it’s a good way to practice strokes, rolls…
Depends on hull/length
Older, longer, displacement hulls will be an easier paddle on flat water than a newer planing hull with lots of rocker. When I paddle my planing hull on flat water I have to push on the opposite foot peg just to keep it going straight.
Depends what your goal is
If you just want the exercise or play and don’t care about speed, it’s pretty decent. In fact, if you try to maintain your regular sea kayak speed in it, it will be quite a workout if you do more than a short jaunt. I once (only once!) paddled two laps of 7.5 to 8 miles each with a couple of other paddlers who were in 18’ and 19’ kayaks. I was surprised at how tired I was after doing that.
By myself, I concentrate on technique. Merely going straight in mine requires paying attention. If there’s even a light breeze, it is a lot of work. I sometimes find it easier to let the wind spin me around a half turn, give it one sweep continuing in the same arc, and then resume forward paddling in the “right” direction.
I really enjoy the thing when I don’t feel like dealing with trailering the sea kayaks. It fits in the back of my pickup truck, strapped in with the tailgate down. It also weighs less than the sea kayaks.
I would never use it for a multiday trip, though–just short day trips.