Japanese researches in Akishima Laboratories working with Osaka University made great strides in the field of writing on water. Why would someone want to do this?
Why ask Why? Try Bud Dry. Theme parks and hotels like the Bellagio would use this to make water shows to entertain their patrons.
The device consists of 50 water wave generators encircling a cylindrical tank 1.6 meters in diameter and 30 cm deep (about the size of a backyard kiddie pool). The wave generators move up and down in controlled motions to simultaneously produce a number of cylindrical waves that act as pixels. The pixels, which measure 10 cm in diameter and 4 cm in height, are combined to form lines and shapes. The device is capable of spelling out the entire roman alphabet, as well as some simple kanji characters.
I wonder if they could adopt this into hot tub technology, drawing out words like "I Love You" and "Don't Stop Don't Stop" when you're in there with your girlie.