Philip hit the nail on the head, water disburses light not focuses it. Sunlight hitting a bottle of water would actually act like a filter and weaken the intensity of the light.
As mentioned the plastic itself may be capable of causing this but again, the plastic is flat not convex so there is no real way of focusing the light to a fine enough point. The corrugated section on the bottle is convex, but too much so and again this would push the light in too many directions and not be focused enough.
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