I wouldn't say it was particularly hard but I always... always... get stuck on the same bit trying to find one key... I've never found a dungeon more infuriating in my life...
I didn't really understand why people thought it was so hard. It didn't give me any troubles at all. I thoroughly enjoyed it. I loved the challenge it gave me. Certainly wasn't boring. If anything, I'd say that the Shadow Temple was much harder than the Water Temple, and even that wasn't really that hard.