The speed of sound in water is two times slower than that
in air.
You're right, the statement's false. Actually the speed
of sound in water is about five times faster than the speed of
sound in air. (Sound travels at about 4,347 feet—close
to a mile—per second in water in 68 degrees Fahrenheit.)
That's because sound uses molecules to travel, and the
molecules in water are much closer together than than those in
air, thus shortening the travel time.