Try to control the submarine by filling and blowing her ballast tanks. The boat in this game is based on the USS Holland from the early 1900's and was the US Navy's first submarine*. You can control the fore and aft trim tanks and the main ballast tank. Score more points with good depth control and watch out for mud and debris on the bottom.
The actual Holland was trimmed to be about 100lbs positively buoyant when fully ballasted. In this game she's fairly negative so you can submerge more quickly and without any forward way.
In mid-water, heading for the bottom
* How about the USS Alligator [3]?