Rick Powers is the modern incarnation of Nostradamus.
I looked everywhere for that key, like wtf...*I'm mad, really mad*...it's like every dungeon in TP is its own version of the OoT Water Temple.
*Goes back playing TP*...lol...this is getting really annoying so I'm going to detail exactly where I am:
I'm in an open area (you can see the sky), it has a lake underneath my uphill path, at the end of my path and to my left is the aggravating locked door, to my closest right is a hanging non-functional magnetic disc, to my furthest right is one of those beam-eye machines, in front of me are 3 monsters constantly shooting arrows at me but they are in an unaccessible platform.
The chest/key you mentioned in the water-spider room, I faced north and saw the underwater chest but it was already opened, so either there should be another chest, or I have no clue what-so-ever.