Ocarina of Time 3DS Water Temple Problem?

OK, I have been in all the places to possibly get keys i think and i went into the room where there is a boulder that rolls in front of you and into the nest room full of crab things. I shot the gold skulltela and found a locked door and the end of the whole thing where the boss key is supposed to be and the door is locked and i don't know the room to go in to get the final small key :(((( Please help!!!