I think it is hard. And my least favorite dungeon in the game, but I have always hated water dungeons
The water temple is just as bad as the one in Twilight Princess (me and my step brother just defeated the frog boss and we had to go to bed) and when we got back on we were lost for hours untill we finally realised we went to every room open to us but the one where we killed the giant frog. After that it was smooth sailing.
It honestly wasn't the hardest temple.
So, it really wasn't that hard and I don't know why people say otherwise.
Seriously, in my opinion all water temples are some of the worst in the game. Water Temple in Ocarina of Time, Great Bay in Majora's Mask, Lakebed Temple in Twilight Princess, and even like Mermai's Cave or Jabu-Jabu's belly in Oracle of Ages. They are always some of the toughest and most agravating dungeons in the game.