So what's interesting is I actually had no idea you could build towers on the underwater sand until I went back and read this comment (and loaded up the game again). I think that might explain some of the difficulty I encountered trying to win. I was trying to win level 2 by only building towers on the 4x4 square in the middle... 😅 so I guess some of my feedback may not be as accurate.
But, with this new information, I might say that the tutorial should outline the sandbars when telling you where to build, or something like that. 😄