In some ways this is similar to the liquid antenna I wrote about in May. [post here] But that antenna was encased in flexible silicone rubber, with a conductive liquid metal inside. There are patents out there for Ionic water antennas. These uniformly fill tubes or hoses with salt water. This antenna is literally just made of water, and uses no casing or enclosure of any kind.
Your tap water is probably not a good conductor of electricity. If it is, it's probably not safe to drink. By comparison, salt water (saline) is pretty conductive. The difference is pretty clear one has sale (Sodium Chloride- NaCl) one does not. But at the atomic level there is a more specific reason. Electrical current is transported by the ions in the saline solution. salt increases both the density and conductivity of water. Seawater is about 3% salt, but it also contains Magnesium (Mg), Calcium (Ca), Potassium (K), Chlorine (Cl), and Sulphate (SO4); of thoseat least Magnesium also improves conductivity.
Conductivity is defined as the ratio of current density to the magnitude of the electric field. That's measured as Siemens per meter. Silver, Gold and Copper are all metals and all very conductive. Let's put that in perspective:
Silver = 6.30 × 106 S/m
Copper = 5.69 × 107 S/m
Gold = 4.52 × 107 S/m
Aluminum = 3.5 × 107 S/m
Water = 10 × 10-6 S/m
Saline = 4.8 S/m
As you can see in the video, it uses a pump to first pull water up to a probe. That "probe" applies current, and shoots a stream of sea water vertically to act as it's antenna. The height of the stream is determined by water pressure, which in turn determines frequency. It makes the antenna completely configurable, so in a marine scenario one might only need one all-purpose seawater antenna for any situation.