Everything fits together quite well, nice music and audio. Continuously giving advice with a voice over could probably get a bit tedious, but I feel you get away with it, because the game isn't very long after all.
I enjoyed the gameplay, but I feel you had to wait around quite a bit before the last two whales get enough water, which doesn't really add much to the challenge anyway, I get that you can make the water go faster by optimizing how you dig the path, but even when I had the number going really fast it still took a while. It definitely would be more interesting to experiment with different ways to get the water to the whale, rather than how to do it as efficiently as possible.
As for the theme, you can only dig so deep. It's more about digging a trench, making it longer and longer, not deeper and deeper.
Everything else that I haven't mentioned is great, very solid entry all in all!