Alright 2 things.
1: Morpha is the easiest boss in the game. I beat her/him in about 70 seconds including opening cutscene.
2: I never found the Water Temple to be the hardest.
First time, it was in the middle. Wasn't easy, wasn't really that difficult. Just took some common sense. Dark Link was the hardest part. I had Biggoron Sword, so I didn't really have to try. I found Shadow Temple excruciatingly easy, and now I can do it pretty well all based on memory (meaning no Lens of Truth)
The hardest Temple for me (timewise, not difficulty) is Forest Temple. Difficulty wise, it's a tie between Forest and Fire. Those aren't ones I particularly love to do. First runthrough, Spirit Temple was hardest.