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.

Super Mario Bros. 3
Water appears in many levels throughout Super Mario Bros. 3, first seen in World 1-5. Water can be found scattered in small pools in the ground, or taking up entire levels. In some cases, water presents enemies such as Boss Bass, Lava Lotus, or Jelectros.

Super Mario Land
In Super Mario Land, water is a staple element of the Muda Kingdom, as it is present in every level of the kingdom. Mario uses the Marine Pop to traverse levels that are submerged in water.

Super Mario World
In Super Mario World, water first appears in Yoshi's Island 4, taking up the bottom part of the screen. It appears in many later levels as well, either placing the whole level under water, or set in only in small sections.