The Weerawila Lake is located along the A2, the Galle Road, but well past Galle and Hambantota, as the road begins to head up to the hills. It's notable because the road cuts right through the lake, giving you stunning views and a quite pleasant drive.
It's near the Saraii Village
hotel, which we reviewed, but besides you may not have much reason to seek this lake out. It is near the Mattala airport. If you are there, enjoy. It's a beautiful place, especially around sunrise or sunset.
Unlike bunds along tanks, there is water on both sides here. It's actually quite strange to drive across, though it's all over in around 15 minutes. It's a nice road, if you're in the neighborhood.