cold water while you prepare each one.
while this is for soft-boiling eggs, i believe you can easily adjust it for hard-boiling.
while for me this is far more complicated, if you're interested you can read all the details in the post "towards the perfect soft boiled egg".
i don't boil eggs enough to keep a feel for the timing... so a few years back, i picked up a handful of these.
they're sold under a few different names, but the idea is the same: sturdy plastic that changes color as it heats. drop it in with the eggs, and pull & chill them all when the color band hits the spot you're looking 