Water

Water is a fluid commonly found in the world generation. It appears as rivers, puddles, and seas. It has little use, other than adding to the landscape appearance. The player can swim across it, but this is dangerous, as it drains the energy bar. If the player keeps swimming after the stamina bar is depleted, he will lose hitpoints until he exits the water or dies.

Mobs cannot enter the water, and thus, cannot swim in it.