Water

Water is a common harmless liquid that frequently serves as an obstacle and level theme. When entered, water can be navigated by swimming through it.

Super Mario Bros. / Super Mario Bros.: The Lost Levels
Water debuts in Super Mario Bros, first appearing in World 2-2. Enemies such as Bloobers and Cheep Cheeps are commonly found in the game's underwater levels. In Super Mario Bros.: The Lost Levels, water first appears in World 2-1, where instead of taking up the entire level, appears in small sections under bridges or between gaps. These small sections of water, however, cannot be swam in, and will cause the player to lose a life if they fall in.