Warm water is an everyday nessecity and for many movie sets with large amount of water, it also needs to be kept warm!
For many shoots, where water is an essential part, it can be nessecary to heat the water so that the actor doesn't freeze. An example could be a recent commercial where it was nessecary to build a snow-landscape in-doors, complete with a frozen lake. In this frozen lake, a dozen winter-bathers we're swimming around and enjoying themselves. As the shoot itself took two whole days, it was nessecary for us to heat the water, so that the actors could confortably stay in it, for so many hours a day!