The thing that most people struggle with in the water temple is finding one of the keys. There's one key that requires you to step on an elevator but get off of it before it moves which reveals a hidden area underneath the elevator. Anyone familiar with the Soulsborne series of games will always know to check under an elevator, but of course Ocarina of Time came out way before that.
If it never occurs to you to check under the elevator, you'll get stuck and wander around for hours trying to find the last key you need which is why, I think, most people hate it.