Very good puzzle game, I really like the concept and the execution is smooth and satisfying (undoing moves is a great feature.) I liked that the water could sometimes be hazardous, it made it so that there was more to the game than just "lava good, water bad."
The one thing I disliked was how formal most solutions were; being exactly one step ahead of failing for so many of the levels really hammered home the fact that most of the time there's ONE solution, so solving a level is the player uncovering that one solution. Because of that, the experience doesn't feel very personal, so much as it feels like they're running along a very deliberate path the developer has laid out for them. This also lends itself to some unintentional hints; when you block one lava path with one turn to spare, it confirms to the player that they've done something correctly, so they know with confidence they'll never have to undo past that point again. Reduction of the solution space makes a lot of the levels easier than they probably should be.
I also disliked how many of the later puzzles had timed blocks on them, this again felt like a cheap way to point the player in the right direction by basically telling them what order they had to do things in.
Overall the game is pretty easy, with level 16 being the only one that really stumped me.