SinkSub Pro consist of 50 individually designed levels. Many levels have a theme or a special feature and by looking at the level description you may get a clue what the level is all about. The current level number is shown at the upper left corner of the game window. Once you have finished a level, you may restart on that level without having to play all the previous levels first. This feature is a form of a save function, except you do not have to think about saving. By starting the game on a higher level you can practice a particular difficult level, or just start at a level that more matches your level of skill. Remember, if you want to be a highscorer, you will earn maximum points by starting from level one.
The skill mode does not affect the number of enemies on a level, only how hard they will be to destroy. By playing the easy skill mode all enemies will be a little less hostile, while selecting the hard mode will make them tougher.
The number of depth charges varies from level to level. In general there will be more charges on the higher levels. Playing the easy skill mode entitles you three extra depth charges on every level.