Such chests need 30mins to respawn. So even if YOU see it, it doesnt say anything about the current status.
Someone could have been there within 30minutes and therefore you cannot open it.
Thanks for your input. You are sure this chest especially is lootable ? When did you (or so else) open it the last time ?
Because - I was there for around 20 minutes in the upper region farming brimstone before, then killing the boss and then searching almost 10 minutes before I found the chest (yes, I killed him the first time). All that time no other player was near.
It is definately lootable, i kill the boss every time i go for brimstone and i do it pretty often not sure if its only 30 min for it to be usable again, could be an hour or so.
Try standing in front of the chest though, it is less likely to open from behind.
I can confirm the chest can definitely be opened, my lady friend goes on three skull boss farming runs regularly, the only thing she ever said about that chest was that it was hard to find the first time.
So with me Killed the Boss, found a key and thought: Where the heck is the chest. Found it finally under water, run around , jumped on it, tried view from all directions and finally gave up, making the ticket here….
Thank you all, sorry I cannot confirm more then one solution, so I chose the first definitive confirmation.