Wow, this is the most fun game ever for pico 8. I love what you did with the fluid dynamics. It makes you think about how the water is flowing, and it is fun to create a big bubble of air that flows to the top.
Could you add a campaign mode to the game with levels. Each level could just be more people to save and faster water and lower dig power, and just keep progressing until the user can't save 75% of the people. The hardest level just doesn't have enough challenge, and I think the progressive difficulty with the rouge like fear of failure as things get harder would turn this into a real nail biter of an experience.