I am an ancient wire twister but have never heard of a commercial use of
water capacitors outside of their application in the sensors used for
measuring level and density of fluids.
Many years ago we helped a youngster build a water tuned crystal radio.
We used a seesaw arrangement for the reservoir and held the tuned
capacitor, a beaker covered with aluminum foil, in a stationary position
(The other conductor for the capacitor was a small diameter tube in the
center of the beaker which contacted the water solution). By putting a
siphon tube between the two containers we were able to tune the
capacitor by changing the waters depth as calibrated by the seesaw's
position. At the radio frequencies of the broadcast band the electrical
Q of the capacitor was very low when compared to an air capacitor,
therefore the tuning was quite broad, which in our area meant that we
picked up several stations at once. We experimented with several
different ionizing materials, e.g., salt, acid, base and none of them
came anywhere close to matching the Q of just plain air. Maybe we should
have used mercury? Memory tells me that we ended up using a much larger
container in order to tune the full range of the broadcast band, but
that could have been yet another project.