okay so i tried everything in my might to find this weapon, let me show you what i learned and researched so far:
1. In reinhardt's lore he went together with his friend brigitte, in surama is a girl named brigitte... no interactions.
2. when winston recalled overwatch, reinhards location is shown in southwest, like faronaar, where the weapon drobbed in the beta.... didnt find anything at all.
3. i went so far to go and farmed myself the bottomfeeder mount to be faster underwater and went ahead and scouted the whole broken isle ocean bit by bit, i found some pretty neat stuff wich does not tie in with the holy hidden skin.
- theres a salty kraken on various locations wich had lvl 99, 103,104, 105, they dont have any interactions and not even a dying animation, but the level scaling seems wierdly off. nothing i could do with them sadly.
- in stormheim on the far northeast where the broken statue of the watcher is (location of one of the orbs to summon roshamul? (forgot the name its the jellyfishmount boss), is a huge stone head face down in the sand, for me it looks a bit like reinhards hair, its a bit farfetched but theres also a armor piece engaved in the huge stone that looks clearly like metal armor, altough i couldnt find anything else except another salty kraken near it might just be part of the jellyfishmount orb statue that flew way far off.
- theres a lot of teethless white sharks around showing off hidden locations and secrets but only one showed the entrace of a cave for the jellyfishboss mount, the otherones seem unfinished....
- found multiple eastereggs like 2x titanic references, a lot of wierd broken temples among other things, strongly doubt they are related to the edict.
4. I tried to check every bit of black rook hold since its apperently the only medieval kind of castle around and reinhard is a german medieval themed knight.
found nothing,
- i didnt checked the dungeon inside, maybe a clue inside the dungeon.... doubt it tho.
5. i checked whole isnland of the watcher, i found some owl statues you can activate and they seem wierd, had to trigger them in certain order to activate a few, but do the fact that reinhard has nothing todo with anykind of owls and i guess its for these shadowsong guys...
6. while deeper into the lore of reinhard, he is named reinhardt wilhelm and theres a paladin that lies defeated in the paladin class hall, he has an ugly hammer but could be a clue... one thing is wierd he seems to somehow gain HP back.... i dont know how but it only wentz from 1% to 12%...
7. Ra-den, secret boss of throne of thunder resembles reinhard quiet a bit, even got his scar, mirrowed on the other eye... could not kill him do a bug and it did not open for me in this ID :/ (strongly doubt they go back to old content for it tho.
8. The lost edict of the watcher was drobbed in faronaar so i strongly believe it was some kind of hint by blizzard to let us search there, theres even a paladin quest line that failed where you rescue one of your followers, searched everywhere (again)... nothing.
9. I killed 12000 demons as holy (gs 840 so it was a bit easier), no drob, maybe its locked behind a certain amount of demons.
10. I found a cave underwater wich is sealed, they reused the big cave entrance a lot so i spotted it after my second swim around the broken isle. my friend gliched in with his demonhunter jump ability, theres a camp and a lot of murlocs spawn named "underlight angler" wich is the same name for the artifakt fishing rod, no drobs or anything related.
conclusion:
i really think its either a bug, not available yet, strongly out of context hidden or simply a raid drob like the shaman set in a raid prior. There are no lion themed buildings, statues or graves. I searched all the oceans and above ground for dragon skeletons to find any indication for a hammer or a questline, nothing.
what i find dissapointing the most is actually theres tons of shipwrecks and secret caves wich are empty, i mean they put so much effort in decoration the oceans, even hidden caves are empty.
I hope this helps you a little bit, i mean you dont have to search the whole ocean now...
will keep you updated if i find something.