This was a great game! I could see this being a really fun full release if you ever decide to expand on it. My only criticism is that it feels like the jam's theme was a bit of an afterthought. While you do have a timer, it feels superfluous, especially when you can turn into a stone on command with "B". I realize you added the ability to turn at will because the game would be obnoxious otherwise, having to wait for the timer every time you wanted to place a stone, so here's my suggestion if you decide to expand the concept. I'd remove the B button as a way to turn into stone and add some way for the player to get time back. Maybe some kind of pickup would increase time remaining, or the timer could pause while you're in water. Alternatively, as the post jam version wouldn't be confined by the theme anymore you could consider removing the timer entirely.
Either way, what you have here is a really enjoyable game and I only harped on about the timer so much because I have nothing else to give criticism on. Great job!