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. It often appears colored light/dark blue or clear, and is seen many times throughout Mario and friends' adventures in the Mushroom Kingdom.

Water can contain a variety of enemies, some of the common ones being Cheep Cheeps, Bloopers, Urchins, Cheep Chomps, and many more.

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.