I don't believe the "watery shadow" is from the aircraft in the picture. The angles seem wrong compared to the other shadows.
The shadow also is shaped differently, and more resembles a Mig 15 or Mig 17. (The location was Russia, right?)
Getting into very iffy ground, my best guess would be that it was an old (and expendable) Mig 17 testing "stealth paint", or even an invisibility device making it hard to detect. The use of an obsolete (but fairly fast) aircraft as a target to test various weapons on would make sense to me.
The other aircraft may have been participating in the experiment or may have been just a coincidence.
Quite interesting, whatever it is. I would also want to know if the cameras involved will clearly record something traveling at the speed of sound.