The Solar Water Lens is a clever way to demonstrate yet another way to collect and focus the sun's power. Many a child has experimented with a holding magnifying glass in sunlight to create a flammable heat source. Unfortunately, while the lens produces high temperatures, it is only projected onto a relatively small area. The limiting factor being the size of the lens. One large enough to produce the required heat for practical cooking would be prohibitively expensive to manufacture. The designers at Green Power Science have experimented with making a water lens, using gravity to give it its parabolic shape.
They admit it is not intended for latitudes very far from the equator. It requires the sun to be primarily overhead, and will produces heat for maybe three hours on a clear day. The focal length for a lens this size, which is about 75-100 cm in diameter, is relatively long. This means the lens must be supported overhead with a structure holding it about 2.5 m above the focus area. So probably not the most practical way employ solar cooking. None the less, the lens impressively produces temperatures of 175°C (350°F) to 345°C (650°F). In a survival situation in the tropics, with some clear plastic sheeting, water, and some creative carpentry, a cooking source would be possible.