There are only a couple of simple rules to remember about water baths. First, that the water you put in them must always be hot, and second that you must never cover them. Why?
While filling a water bath with lukewarm or even cool water seems to make sense (slow warming, right?), it actually leads to broken custard, since the surface of the custard that’s exposed to the 325-degree air overcooks by the time the water heats up to cooking temperature. That said it pays to keep a large pot of water simmering wile you prepare your custard. Fill a water bath by sliding a rack out of your oven part-way, placing the roaster with the custards on it, then carefully pouring in the almost-boiling water.
And as mentioned, never cover it. A covered bath prevents evaporation, creating a steamy environment in which the water eventually boils (and you definitely don’t want that).