You can get it (relatively) early on
I don’t typically play Fallout with a melee build in mind (quite the opposite, actually; I love sniper rifles), but this power fist served me well for a good chunk ofFallout 4.
First off, look at it. That’s some gnarly shit! It’s like a makeshift Wolverine claw.
Second, this thing works wonders against the AI. So long as you have enough defense to run up to enemies, it’s easy to overwhelm them with repeated punches. They might even drop their weapon, or get a crippled limb. During the first part of your adventure, when ammo for your best guns will still be scarce and expensive, it’s reassuring to have a melee option to fall back on.
So where is it?
You can find this power fist in the pond near Boston Common.
Specifically, on the map, it’s here. (And since that’s a mess to look at, here’s a close up.)
If you go into the water, something will happen. Let’s just say you’ll want to bring a lot of firepower — explosives, if you can spare any, and probably power armor to be on the safe side.
Good luck!