Sound travels fastest in water at its highest density, which occurs at its point of maximum compression. This happens around 4° Celsius (39.2° Fahrenheit).

Here's why:

* Compressibility: Sound travels through a medium by causing vibrations in the molecules of that medium. The more compressible a medium is, the slower sound travels.

* Density: Water's density increases as it cools down from room temperature to 4°C. This means the molecules are closer together, making it harder to compress and thus allowing sound to travel faster.

* Expansion: As the temperature drops below 4°C, water starts to expand (due to the formation of ice crystals), becoming less dense. This makes it easier to compress and slows down the speed of sound.

So, while sound generally travels faster in warmer water compared to very cold water, it reaches its peak speed at that sweet spot of 4°C.