Haha the infamour Water Temple... Has anyone noticed that in *any* game the Water-based levels are always the worst? Haha... Oh dear...
This goes back some time for me (I just finished the game yesterday) but... All I can really advise you to do is check your map and compass, if you've found the treasure that may be an integral part of finding the key too.
That's one formula I've observed through the game, the dungeon treasure is usually instrumental in accessing all areas of the dungeon and collecting everything there is to collect, so keep your eyes open
One more thing... Look upwards, you'll never know what you will find above your head if you look