My wife has a cloth tube (for lack of a better description) that is filled with corn that we zap in the microwave when we are cold. She puts it over her head sometimes if she has a headache.
I will often times warm that up and put that between my legs for a good 15 minutes before hand. I’ll just sit and watch TV with that warming me up, and then I’ll go to my PE. Warm water I’m sure will work, but it probably can’t effectively heat up everything that should be, like the base where the ligaments are. Just a thought from a newbie who might not know what he’s talking about! :)