Sony put out a teaser video that proclaims "The time to Demand Great is coming." The video features a stylized flip clock that reveals four clues, which we've combined into the image below.
The last one is easy enough September 3 is when the Sony IFA event will take place. The H2O bit is also easy, the new Xperias Xperia Z3, Xperia Z3 Compact and Xperia Z3 Tablet (Compact?) will be water-resistant just like their predecessors.
"7MM" could be referring to a thickness, the current Xperia Z2 phone is 8.2mm thick, the Xperia Z2 Tablet is actually thinner, 6.4mm. This only leaves "Cu" unexplained but it could be as simple as "see you."
Here's the video teaser. At 0:40 you can see the silhouettes of the three upcoming devices.