It was really difficult to see the difference between paths-hidden-by-lasers that you could walk on and those you fell through, especially when there was only one type on a screen; Also, I found using shallow water and waterfalls as kill-blocks rather irritating. Ok, once you knew it got better, but it put me in a bad mood which made the puzzles frustrating rather than challenging.
Also, lack of ambiance is irritating. Water makes a noise!
Sorry, like I say those surprise-deaths until I learnt my way around put me in a bad mood. The challenges, however, were good. If anything it's a shame this level was so short so that there couldn't be more variation and development. Maybe a second area below the first with the other two keys?
The number of save-points was nice and forgiving; on the whole if learning what was safe and what was solid hadn't set me in a bad mood it would have been a nice little challenge. Better design on the narrow walkways to show what was solid and what was not (such as black outlines on the solid ones) would have been a big improvement. And using the actual deadly water and other traps to kill the player rather than mocking up your own dangers would have helped, too.
Plus some sound would have helped a lot.
All together the challenges were good but the confusing design let it down.