A quick and (hopefully) easy tower simulator set underwater. About 10 minutes to complete. I recommend playing at least until you have 10 population. (Beat the game at 30.) Control with the mouse and Backspace. F10 to change resolution. F11 for fullscreen. If you get stuck check the README.

How it fits the theme: You create an airscraper (underwater skyscraper) out of coral and create a city-like ecosystem that fish can live and shop in.