The temperature in a beehive averages 95º F but can get up to 140º F. Putting honey in hot water can work, but then the water gets cold and you've got to add more hot and on and on.
We've found success a couple of different ways -- place the honey in a warm sunny location (south facing window) or create a 'warm room' or a 'warm box' and leave the honey there for a few days until it re-liquifies. It takes patience. As well at how much it has crystallized, it also can depend on the type of honey. Lighter honey will tend to crystallize quicker and be hard to re-liquify. Darker honeys will tend to re-liquify more easily. Good luck!