I bought a ball valve to install a shut off on my hot water heater. However, it says that it is for LP gas. Will it work for water also?

While it might work for you I would not recommend using that valve for water. Believe me, if the manufacturer thought it would work both ways he'd advertise it that way.

Some ball valves are manufactured for multi use. If it says it can also be used for water then it will be OK. If it does not, then don't