A method to realize low-cost, optically transparent, flexible, and unidirectional antennas is presented in this article. The key to the method is making a flexible transparent reflector made using a new method-injection of pure water into a flexible transparent cavity that is made out of polydimethylsiloxane (PDMS) polymer and integrating the flexible reflector with a flexible dipole made of transparent conducting mesh. This method is simple and inexpensive. Hence, it is useful for large-scale production of transparent, flexible, robust, and low-cost antennas as well as RF/microwave components for wearable body-area networks and other such systems that require flexible and transparent components. To validate the method, an antenna operating in the 2.45 GHz band was designed, fabricated, and tested under various conditions. Its measured gain is 3.2 dBi and efficiency is 51%. To the best of our knowledge, this is the first water-based transparent flexible antenna. Due to the shielding effect of the water-based reflector, it produces a low specific absorption rate (SAR) in the human body when it is worn, as expected from a unidirectional antenna. Furthermore, it is investigated and confirmed that the use of pure water in the reflector makes the antenna significantly more efficient as opposed to salt water.