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.