To elaborate on @Janus Bahs Jacquet's important comment here:
In Mandarin, for something to be "open" (开) more generally refers to it being in an active state, or in a state where it is ready for use. E.g. to "turn on the computer" (iirc) is "开电脑" - bring it into the state where it can be used. To understand how this applies to the case of water, you have to remember that Chinese always boil their water prior to use - which makes sense if you think about sanitation and germs. Water that has not been boiled is thus not "ready for use", while that which has, is now ready for use: it is 开, open, in the active state, the state where it is ready for use, i.e. for drinking, since the germs that would have otherwise made it hazardous to drink have been destroyed.
Note, by the way, that this phrase specifically refers to boiled, not simply "hot", water. Hot water really is just 热水, as you would expect.