Yes, I think the Water Temple was pretty difficult. Although, I do not think it is as hard and frustrating as people seem to think. With common sense, (checking every room for small keys, defeating every enemy, clearing all puzzles, etc.) this temple can be completed whilst only a little bit of frustration. My only problem with it was having to change the water level when I missed a small key. I found that pretty tedious and annoying. It's difficult at first glance, but once you get the hang of it, it's really not that bad; so I'll vote "Pretty challenging."